My constructor sets the attribute, so why is EFcore complaining?

 try
 {
     int OnaId = await HttpHandler.GetOnaIdAsync(client, userInfoUri, token);
     var user = await _userService.GetUserByOnaIdAsync(OnaId);
     if (user == null)
     {
         user = await _userService.CreateUserByOnaIdAsync(OnaId);
         //return NotFound("User not found. Call the \"Ona/Token\" Get/Put methods in api/Users first");
     }
     var upMedia = new UpMedia(upMediaDto.Type, user, upMediaDto.Bucket, upMediaDto.Path);
     var createdUpMedia = await _upMediaService.CreateUpMediaAsync(upMedia);
     return Ok(UpMediaMapper.ToUpMediaDto(createdUpMedia));
 ...

   public class UpMedia : BaseUpload
   {
       [Key]
       [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
       public Guid Id { get; set; }
       [Required]
       public MediaType Type { get; set; } // Metadata

       // Navigation property        
       public required Guid OwnerId { get; set; }
       public required User Owner { get; set; }  // must always be present

       public ICollection<Post> Posts { get; set; }

       public UpMedia() : base()
       {
           Posts = new HashSet<Post>();
       }
       public UpMedia(MediaType Type, User Owner, string Bucket, string Path) : base(Bucket, Path)
       {
           this.Type = Type;
           this.OwnerId = Owner.Id;
           this.Owner = Owner;
           this.Posts = new HashSet<Post>();
       }

   }