Create user by identity ASP.NET Core 5 - asp.net

Question

I am trying to send user to database using identity and sqlite, I am using swagger to contact api. When I click execute my request but I get the following error:
System.TypeInitializationException: The type initializer for 'Microsoft.EntityFrameworkCore.Query.QueryableMethods' threw an exception.
System.InvalidOperationException: Sequence contains more than one matching element
at System.Linq.ThrowHelper.ThrowMoreThanOneMatchException()
at System.Linq.Enumerable.TryGetSingle[TSource](IEnumerable1 source, Func2 predicate, Boolean& found)
at System.Linq.Enumerable.Single[TSource](IEnumerable1 source, Func2 predicate)
at Microsoft.EntityFrameworkCore.Query.QueryableMethods..cctor()
--- End of inner exception stack trace ---
at Microsoft.EntityFrameworkCore.Query.QueryableMethods.get_FirstOrDefaultWithPredicate()
at Microsoft.EntityFrameworkCore.EntityFrameworkQueryableExtensions.FirstOrDefaultAsync[TSource](IQueryable1 source, Expression1 predicate, CancellationToken cancellationToken)
at Microsoft.AspNetCore.Identity.EntityFrameworkCore.UserOnlyStore6.FindByNameAsync(String normalizedUserName, CancellationToken cancellationToken) at Microsoft.AspNetCore.Identity.UserManager1.FindByNameAsync(String userName)
at Microsoft.AspNetCore.Identity.UserValidator1.ValidateUserName(UserManager1 manager, TUser user, ICollection1 errors) at Microsoft.AspNetCore.Identity.UserValidator1.ValidateAsync(UserManager1 manager, TUser user) at Microsoft.AspNetCore.Identity.UserManager1.ValidateUserAsync(TUser user)
at Microsoft.AspNetCore.Identity.UserManager`1.CreateAsync(TUser user)
at DateApp.Controllers.HomeController.Register() in /Users/michal/Projects/DateApp/DateApp/Controllers/HomeController.cs:line 78
I am expecting to push new user to my sqlite database.
My code - controller:
[HttpPost("add")]
public async Task<IActionResult> Register()
{
if (ModelState.IsValid)
{
var user = new IdentityUser
{
UserName = "admin",
Email = "aadmin#gmail.com"
};
//create without password
var res = await _userManager.CreateAsync(user);
if (!res.Succeeded)
{
var exceptionText = res.Errors.Aggregate("User Creation Failed - Identity Exception. Errors were: \n\r\n\r", (current, error) => current + (" - " + error + "\n\r"));
throw new Exception(exceptionText);
}
}
return Ok();
}
DbContext:
using System;
using Microsoft.AspNetCore.Identity;
using Microsoft.EntityFrameworkCore;
using Microsoft.AspNetCore.Identity.EntityFrameworkCore;
using DateApp.Models;
namespace DateApp.Db
{
public class AppDBContext : IdentityDbContext
{
private readonly DbContextOptions _options;
public AppDBContext(DbContextOptions options) : base(options)
{
_options = options;
}
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
base.OnModelCreating(modelBuilder);
}
}
}