Why laravel 6 auth returns false after redirecting by using custom guard?

The problem is in your routes file:

Route::group(['namespace' => $namespace,'prefix' => 'admin',  'middleware' => ['auth']  ], function () {    Route::get('dashboard', 'DashboardController@index')->name('dashboard');});

You are using the default guard with auth middleware. After you are logged in with admin guard you may not be logged in by your default web guard. That is why it fails and tries to redirect you to login page:

When I try to login, after submitting valid details it does not redirecting me to dashboard, nothing happening. Also when I try for open forcefully /dashboard it take me to login page.

Instead, you should specify in your group that you are using the admin guard:

Route::group(['namespace' => $namespace,'prefix' => 'admin',  'middleware' => ['auth:admin']], function () {    Route::get('dashboard', 'DashboardController@index')->name('dashboard');});

However, you already specified in your DashboardController to use $this->middleware('auth:admin');, so there is no need to specifiy it in the route group again. The following is enough and reduces the likelihood to create an error:

Route::group(['namespace' => $namespace,'prefix' => 'admin'], function () {    Route::get('dashboard', 'DashboardController@index')->name('dashboard');});

An extraction sample of the how you should define your admin model:

 // app/Admin.php<?phpnamespace App;use Illuminate\Notifications\Notifiable;use Illuminate\Foundation\Auth\User as Authenticatable;class Admin extends Authenticatable{    use Notifiable;    protected $guard = 'admin';    protected $fillable = [        'name', 'email', 'password',    ];    protected $hidden = [        'password', 'remember_token',    ];}

For more on multiple authentications guards see: How to use multiple authentication guards

Auth::guard('admin')->attempt($request->only('email','password') its returning true or false? If returning false then maybe toy didnt hashed your passwordTry add this in your Model

 public function setPasswordAttribute($password)    {        $this->attributes['password'] = Hash::make($password);    }