ASP.NET Core Web API - The name 'WebApplication' does not exist in the current context WebApi

In ASP.NET Core-6 Web API, I have this code in Program.cs of the WebApi Project:

WebApi Program.cs:

var builder = WebApplication.CreateBuilder(args);

// Add services to the container.

builder.Services.AddControllers();
// Learn more about configuring Swagger/OpenAPI at https://aka.ms/aspnetcore/swashbuckle
builder.Services.AddEndpointsApiExplorer();
builder.Services.AddSwaggerGen();

var app = builder.Build();

// Configure the HTTP request pipeline.
if (app.Environment.IsDevelopment())
{
    app.UseSwagger();
    app.UseSwaggerUI();
}

app.UseHttpsRedirection();

app.UseAuthorization();

app.MapControllers();

app.Run();

I got this error:

Error CS0103 The name 'WebApplication' does not exist in the current context WebApi

How do I resolve this?

Thanks