1
Inconsistent accessibility: "Iclientmanager" parameter type is less accessible than "Clientecontroller.Clientcontroller(Iclientmanager, Usermanager)
Controller:
public class ClientController : Controller
{
    private readonly IClientManager _clientManager;
    private readonly UserManager<ApplicationUser> _userManager;
    public ClientController(   <-  Erro está aqui
        IClientManager clientManager,
        UserManager<ApplicationUser> userManager)
    {
        _clientManager = clientManager;
        _userManager = userManager;
    }
}
Iclientmanager:
internal interface IClientManager : IDisposable
{
    Task<ApplicationClient> CreateClientAsync(ApplicationClient client);
}
Startup:
services.AddScoped<IClientManager, ClientManager>();
Clientmanager:
public class ClientManager : IClientManager
{
    private ApplicationDbContext _context;
    public ClientManager(ApplicationDbContext context)
    {
        _context = context;
    }
    public async Task<ApplicationClient> CreateClientAsync(ApplicationClient client)
    {
        var result = await _context.Clientes.AddAsync(client);
        if (result.State == EntityState.Added)
        {
            _context.SaveChanges();
        }
        return result.Entity;
    }
}
Now gave the following message
Mais de um modificador de proteção– Matheus
sorry there was an error in the reply, already corrected
– Ricardo Pontual
Now it worked perfectly
– Matheus