Добавил lazyLoading

Payments/Payment.WebApi/Controllers/UserController.cs

@@ -9,7 +9,7 @@ namespace Payment.WebApi.Controllers;
 
 [ApiController]
 [Route("Users/[action]")]
-[Produces("applications/json")]
+[Produces("application/json")]
 public class UserController : Controller
 {
     private readonly IUserRepository _userRepository;

Payments/Payment.WebApi/Infrastructures/Databases/PaymentDbContext.cs

@@ -12,6 +12,11 @@ public class PaymentDbContext : DbContext
     {
     }
 
+    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
+    {
+        optionsBuilder.UseLazyLoadingProxies();
+    }
+
     protected override void OnModelCreating(ModelBuilder modelBuilder)
     {
         base.OnModelCreating(modelBuilder);

Payments/Payment.WebApi/Models/DbModels/User.cs

@@ -8,7 +8,7 @@ public class User
     public string? MiddleName { get; set; }
     public string Email { get; set; }
     public int Age { get; set; }
-    public List<UserBalance> Balances { get; set; }
+    public virtual List<UserBalance> Balances { get; set; }
     public bool IsActive { get; set; }
 
     public User()

Payments/Payment.WebApi/Models/DbModels/UserBalance.cs

@@ -6,5 +6,5 @@ public class UserBalance
     public decimal Amount { get; set; }
     public string Currency { get; set; }
     public int UserId { get; set; }
-    public User User { get; set; }
+    public virtual User User { get; set; }
 }
\ No newline at end of file

Payments/Payment.WebApi/Payment.WebApi.csproj

@@ -11,6 +11,7 @@
           <PrivateAssets>all</PrivateAssets>
           <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
         </PackageReference>
+        <PackageReference Include="Microsoft.EntityFrameworkCore.Proxies" Version="6.0.7" />
         <PackageReference Include="Npgsql.EntityFrameworkCore.PostgreSQL" Version="6.0.5" />
         <PackageReference Include="Swashbuckle.AspNetCore" Version="6.2.3" />
         <PackageReference Include="Swashbuckle.AspNetCore.Annotations" Version="6.4.0" />