Method Illuminate\Auth\RequestGuard::attempt does not exist while testing Laravel Passport restful api

I am using Laravel-8 for restful api, and it it I have Laravel Passport Authentication:

public function login(Request $request)
{
    $credentials = [
        'email' => $request->email,
        'password' => $request->password
    ];

    if (auth()->attempt($credentials)) {
        $token = auth()->user()->createToken('MyApp')->accessToken;
        return response()->json(['token' => $token], 200);
    } else {
        return response()->json(['error' => 'UnAuthorised'], 401);
    }
}

auth.api

'defaults' => [
    'guard' => 'api',
    'passwords' => 'users',
],

'guards' => [
    'web' => [
        'driver' => 'session',
        'provider' => 'users',
    ],

    'api' => [
        'driver' => 'passport',
        'provider' => 'users',
        'hash' => false,
    ],
],

I have this route in api.php:

Route::group(['prefix' => 'v1/admin', 'as' => 'admin.', 'namespace' => 'Api\v1\Admin\', 'middleware' => ['api']], function () {
  Route::post('login', 'AdminLoginController@login');
});

When I tried to test on POSTMAN using POST Request, I got this error:

BadMethodCallException: Method Illuminate\Auth\RequestGuard::attempt does not exist. in file

How do I resolve this?

Thanks