To com essa exceção desde o curso passado(baixei o código de vocês e a exceção continua.).

LoginService

using FluentResults;
using Microsoft.AspNetCore.Identity;
using System.Linq;
using UsuariosApi.Data.Requests;
using UsuariosApi.Models;
using UsuariosApi.Services;

public class LoginService
    {
        private SignInManager<IdentityUser<int>> _signInManager;
        private TokenService _tokenService;

        public LoginService(SignInManager<IdentityUser<int>> signInManager,
            TokenService tokenService)
        {
            _signInManager = signInManager;
            _tokenService = tokenService;
        }

        public Result LogaUsuario(LoginRequest request)
        {
            var resultadoIdentity = _signInManager
                .PasswordSignInAsync(request.Username, request.Password, false, false);
            if (resultadoIdentity.Result.Succeeded)
            {
                var identityUser = _signInManager
                    .UserManager
                    .Users
                    .FirstOrDefault(usuario => 
                    usuario.NormalizedUserName == request.Username.ToUpper());
                Token token = _tokenService.CreateToken(identityUser);
                return Result.Ok().WithSuccess(token.Value);
            }
            return Result.Fail("Login falhou");
        }
    }

LoginRequest

using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.Linq;
using System.Threading.Tasks;

namespace UsuariosApi.Data.Requests
{
    public class LoginRequest
    {
        [Required]
        public string Username { get; set; }
        [Required]
        public string Password { get; set; }
    }
}

LoginController

using FluentResults;
using Microsoft.AspNetCore.Mvc;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using UsuariosApi.Data.Requests;
using UsuariosApi.Services;

namespace UsuariosApi.Controllers
{
    [ApiController]
    [Route("[controller]")]
    public class LoginController : ControllerBase
    {
        private LoginService _loginService;

        public LoginController(LoginService loginService)
        {
            _loginService = loginService;
        }

        [HttpPost]
        public IActionResult LogaUsuario(LoginRequest request)
        {
            Result resultado = _loginService.LogaUsuario(request);
            if (resultado.IsFailed) return Unauthorized(resultado.Errors);
            return Ok(resultado.Successes);
        }
    }
}