LoginAsync.cs

public async Task<AuthenticationResult> LoginAsync(string email, string password)
{
    var user = await _userManager.FindByEmailAsync(email);

    if (user == null)
    {
        return new AuthenticationResult
        {
            Errors = new[] {"User does not exist"}
        };
    }

    var userHasValidPassword = await _userManager.CheckPasswordAsync(user, password);

    if (!userHasValidPassword)
    {
        return new AuthenticationResult
        {
            Errors = new[] {"User/password combination is wrong"}
        };
    }

    return await GenerateAuthenticationResultForUserAsync(user);
}