Why does it say "The field is required" in postman even if it's nullable?

I have connected my asp.net core project to MS SQL server and am trying to test my APIs after writing the controllers. Here are 2 of the data models that I am trying to use.

 public partial class Content
    {
        public Guid CCuid { get; set; }
        public Guid CPuid { get; set; }
        public string? CContents { get; set; }
        public decimal? CAmount { get; set; }
        public string? CCheckNumber { get; set; }
        public int? CQuantity { get; set; }
        public string? CNotes { get; set; }
        public DateTime CDateProcessed { get; set; }
        public string? CUserName { get; set; }
        public virtual Vompackage? CPu { get; set; } 
    }


 public partial class Package
      {
        public Package()
        {
            Contents = new HashSet<Content>();
        }
    
        /// <summary>
        /// Unique Package Identifier
        /// </summary>
        public Guid PPuid { get; set; }
        public Guid PSuid { get; set; }
        public string? PTrackingNumber { get; set; }
        public string? PBolnumber { get; set; }
        public string? PProductCode { get; set; }
        public int? PQuantity { get; set; }
        public int? PPallets { get; set; }
        public int? PBoxes { get; set; }
        public string? PNotes { get; set; }
        public DateTime PDateEntered { get; set; }
    
        public virtual Vomshipment? PSu { get; set; }
        public virtual ICollection<Content> Contents { get; set; }
    }
    }

Now I am trying to add a content using postman. When I tried to add the following data,

{
    "cAmount": 2332,
    "cCheckNumber": "",
    "cQuantity": 133,
    "cNotes": "thank u ",
    "cDateProcessed": "2020-12-02T13:40:47.207",
    "cUserName": "ztinsley",
    "CPu": null
    }
         

It gives 400 response, and

"errors": {
    "CPu": [
        "The CPu field is required."
    ]

I used null! to make CPu nullable. And when I tested GET method to pull all the contents, it says "cPu": null for every data. Why is it having trouble adding a new data? I also tried to add options => options.SuppressImplicitRequiredAttributeForNonNullableReferenceTypes = true in AddControllers(), but the it gives me 500 response. Please help!

P.S. I have attached my controller to see if I made any mistakes.

   [Produces("application/json")]
        [ApiController]
        [Route("api/content")]
        public class ContentController : Controller
        {
            private readonly IContentRepository _contentReporitory;
    
            public ContentController(IContentRepository contentReporitory)
            {
                _contentReporitory = contentReporitory;
            }
   [HttpGet("{ccuid}")]
        public async Task<ActionResult<Content>> GetContentById(Guid ccuid)
        {
            try
            {
                var result = await _contentReporitory.GetContentById(ccuid);

                if (result == null)
                {
                    return BadRequest();
                }

                return result;
            }
            catch (Exception)
            {
                return StatusCode(StatusCodes.Status500InternalServerError,
                    "Error retrieving data from the database");
            }
        } 
            [HttpPost("addcontent")]
            public async Task<ActionResult<Content>> AddContent([FromBody]Content content)
            {
                try
                {
                    if (content == null)
                        return BadRequest();
    
                    var newContent = await _contentReporitory.AddContent(content);
    
                    return CreatedAtAction(nameof(GetContentById),
                        new { id = newContent.CCuid }, newContent);
    
    
                } catch (Exception)
                {
                    return StatusCode(StatusCodes.Status500InternalServerError);
                }
            }
    
        }
    }

My repository:

    public class ContentRepository : IContentRepository
    {
        private readonly OperationsContext _operationsContext;

        public ContentRepository(OperationsContext operationsContext)
        {
            _operationsContext = operationsContext;
        }

        public async Task<Content> AddContent(Content content)
        {
            var result  = await _operationsContext.Contents.AddAsync(content);
            await _operationsContext.SaveChangesAsync();
            return result.Entity;
        }


    }
}