how to solve 401 unauthorized error in postman when calling a .Net API

Since I am not clear about your specific code implementation, I wrote a demo here, which is an example of generating token from user login to access permission API. You can refer to it, maybe it will help you a little:

First,open the appsettings.json file and change the section named Jwt:

"Jwt": {
    "Issuer": "testUser",
    "Audience": "user",
    "Key": "this is my custom Secret key for authnetication"
  }

Enable the JWT authentication scheme and swagger authorization configuration when the configuration starts, the entire code is as follows:

using Microsoft.AspNetCore.Authentication.JwtBearer;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using Microsoft.IdentityModel.Tokens;
using Microsoft.OpenApi.Models;
using System;
using System.Collections.Generic;
using System.IO;
using System.Reflection;
using System.Text;
using WebApplication129.Controllers.conf;

namespace WebApplication129
{
    public class Startup
    {
        public Startup(IConfiguration configuration)
        {
            Configuration = configuration;
        }

        public IConfiguration Configuration { get; }

        // This method gets called by the runtime. Use this method to add services to the container.
        public void ConfigureServices(IServiceCollection services)
        {
            //services.AddSingleton<IConfiguration>(Configuration);
            services.AddAuthentication(opt => {
                opt.DefaultAuthenticateScheme = JwtBearerDefaults.AuthenticationScheme;
                opt.DefaultChallengeScheme = JwtBearerDefaults.AuthenticationScheme;
            })
     .AddJwtBearer(options =>
     {
         options.TokenValidationParameters = new TokenValidationParameters
         {
             ValidateIssuer = true,
             ValidateAudience = true,
             ValidateLifetime = true,
             ValidateIssuerSigningKey = true,
             ValidIssuer = Configuration["Jwt:Issuer"],
             ValidAudience = Configuration["Jwt:Audience"],
             IssuerSigningKey = new SymmetricSecurityKey(Encoding.UTF8.GetBytes(Configuration["Jwt:Key"]))
         };
     });
            services.AddControllers(options =>
            {
                options.Conventions.Add(new GroupingByNamespaceConvention());
            });

            services.AddSwaggerGen(config =>
            {   
                config.AddSecurityDefinition("Bearer", new OpenApiSecurityScheme
                {
                    Description =
        "JWT Authorization header using the Bearer scheme. \r\n\r\n Enter 'Bearer' [space] and then your token in the text input below.\r\n\r\nExample: \"Bearer 12345abcdef\"",
                    Name = "Authorization",
                    In = ParameterLocation.Header,
                    Type = SecuritySchemeType.ApiKey,
                    Scheme = "Bearer",
                    BearerFormat="JWT"
                });

                config.AddSecurityRequirement(new OpenApiSecurityRequirement()
{
    {
        new OpenApiSecurityScheme
        {
            Reference = new OpenApiReference
            {
                Type = ReferenceType.SecurityScheme,
                Id = "Bearer"
            },
            Scheme = "oauth2",
            Name = "Bearer",
            In = ParameterLocation.Header,

        },
        new List<string>()
    }
});


                var titleBase = "Test API";
                var description = "This is a Web API for Test operations";
                var TermsOfService = new Uri("https://xxxxxx");
                var License = new OpenApiLicense()
                {
                    Name = "MIT"
                };
                var Contact = new OpenApiContact()
                {
                    Name = "Test",
                    Email = "Test@hotmail.com",
                    Url = new Uri("https://xxxxxx")
                };

                config.SwaggerDoc("v1", new OpenApiInfo
                {
                    Version = "v1",
                    Title = titleBase + " v1",
                    Description = description,
                    TermsOfService = TermsOfService,
                    License = License,
                    Contact = Contact
                });

                config.SwaggerDoc("v2", new OpenApiInfo
                {
                    Version = "v2",
                    Title = titleBase + " v2",
                    Description = description,
                    TermsOfService = TermsOfService,
                    License = License,
                    Contact = Contact
                });

                var xmlFile = $"{Assembly.GetExecutingAssembly().GetName().Name}.xml";
                var xmlPath = Path.Combine(AppContext.BaseDirectory, xmlFile);
                config.IncludeXmlComments(xmlPath);
            });
        }

       
        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
        {
            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
            }
            app.UseSwagger();
            app.UseSwaggerUI(config =>
            {
                config.SwaggerEndpoint("/swagger/v1/swagger.json", "Test v1");
                //config.SwaggerEndpoint("/swagger/v2/swagger.json", "Test v2");
            });
            app.UseHttpsRedirection();

            app.UseAuthentication();

            app.UseRouting();

            app.UseAuthorization();

            app.UseEndpoints(endpoints =>
            {
                endpoints.MapControllers();
            });
        }
    }
}

Log in and generate the jwt part as follows. Since I did not use it with a database, I customized a user:

Model:

public class Usuario
    {
        public string NomeUsuario { get; set; }
        public string Senha { get; set; }
    }

Controller:

using Microsoft.AspNetCore.Mvc;
using Microsoft.Extensions.Configuration;
using Microsoft.IdentityModel.Tokens;
using System;
using System.Collections.Generic;
using System.IdentityModel.Tokens.Jwt;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using WebApplication129.Model;

namespace WebApplication129.Controllers.V1
{
    [Route("api/[controller]")]
    [ApiController]
    public class SegurancaController : Controller
    {
        private IConfiguration _config;
        public SegurancaController(IConfiguration Configuration)
        {
            _config = Configuration;
        }
        [HttpPost]
        [Route("login")]
        public IActionResult Login([FromBody] Usuario loginDetalhes)
        {
            bool resultado = ValidarUsuario(loginDetalhes);
            if (resultado)
            {
                var tokenString = GerarTokenJWT();
                return Ok(new { token = tokenString });
            }
            else
            {
                return Unauthorized();
            }
        }
        private string GerarTokenJWT()
        {
            var issuer = _config["Jwt:Issuer"];
            var audience = _config["Jwt:Audience"];
            var expiry = DateTime.Now.AddMinutes(120);
            var securityKey = new SymmetricSecurityKey(Encoding.UTF8.GetBytes(_config["Jwt:Key"]));
            var credentials = new SigningCredentials(securityKey, SecurityAlgorithms.HmacSha256);
            var token = new JwtSecurityToken(issuer: issuer, audience: audience,
expires: expiry, signingCredentials: credentials);
            var tokenHandler = new JwtSecurityTokenHandler();
            var stringToken = tokenHandler.WriteToken(token);
            return stringToken;
        }
        private bool ValidarUsuario(Usuario loginDetalhes)
        {
            if (loginDetalhes.NomeUsuario == "TestName" && loginDetalhes.Senha == "TestPwd")
            {
                return true;
            }
            else
            {
                return false;
            }
        }
    }
}

Test and verify API:

 [ApiController]
[Route("api/v1/[controller]")]
public class HomeController : ControllerBase
{
    /// <summary>
    /// Add the description information you need    
    /// </summary>
    /// 
    /// 
    [Route("test")]
   [HttpGet]
    
    public string Test( int userID)
    {
        return "v1 test";
    }

    [Route("list_data")]
    [HttpGet]
    [Authorize]
    public Object  Data()
    {
        User user = new User();
        user.id = 1;
        user.userName = "Test";
        user.email = "test@xxx.com";
        user.address = "testAddress";

        return user;
    }

}

The above shows two APIs, one requires authorization and the other does not require authorization to access.

Resutl: