.net core Identity, getting meaningful login failed reason

NotAllowed means either the Email or Phone Number have't been confirmed (and confirmation is required). You can check this explicitly with something like the following (assuming you have a UserManager instance from DI):

await _userManager.IsEmailConfirmedAsync(user);
await _userManager.IsPhoneNumberConfirmedAsync(user);

To use either of those two functions, you'll need the user:

var user = await _userManager.FindByNameAsync(loginViewModel.UserName);

To determine whether it was the username or the password that failed, you'll need to first check IsLockedOut, IsNotAllowed and RequiresTwoFactor. If all of these return false, the username or password is incorrect. In order to determine which of these is the problem, you can check the return value from await _userManager.FindByNameAsync(user). Here's a complete example:

var signinResult = await _signInManager.PasswordSignInAsync(
    loginViewModel.UserName, loginViewModel.Password, false, false);

var user = await _userManager.FindByNameAsync(loginViewModel.UserName);

if (signinResult.IsNotAllowed)
{
    if (!await _userManager.IsEmailConfirmedAsync(user))
    {
        // Email isn't confirmed.
    }

    if (!await _userManager.IsPhoneNumberConfirmedAsync(user))
    {
        // Phone Number isn't confirmed.
    }
}
else if (signinResult.IsLockedOut)
{
    // Account is locked out.
}
else if (signinResult.RequiresTwoFactor)
{
    // 2FA required.
}
else
{
    // Username or password is incorrect.
    if (user == null)
    {
        // Username is incorrect.
    }
    else
    {
        // Password is incorrect.
    }
}

PasswordSignInAsync returns only a generic failure if the username/password is incorrect. This is because you really shouldn't be exposing which is wrong in the first place. If you explicitly tell the end-user that the password is wrong, then they now know that the username is good. If this is a malicious user, you've halved their work. Now they can just brute-force the password.

If you do want this information, though, simply attempt to get the user by the username first:

var user = await _userManager.FindByNameAsync(loginViewModel.UserName);
if (user == null)
{
    // username is invalid
}

// Now attempt to login with password.

In my case, it was one of the most silly reason, I seeded an Admin user and forgot to mark EmailConfirmed to true and had services.AddDefaultIdentity<IdentityUser>(options => options.SignIn.RequireConfirmedAccount = true) in my ConfigureServices method. Make sure you have not required Confirmed Account while login or if the user you are trying to login has his email confirmed.