Route::get('login', function () { return response(['message' => "Login test"], 401);
})->name('login');
//Just name your preffered route 'login'

Use Postman and set the Header `Accept: application/json` otherwise Laravel
Passport would never know it's an API client and thus redirect to a /login
page for the web.

You're trying to redirect to a named route whose name is login, but you have
no routes with that name:
Route::post('login', [ 'as' => 'login', 'uses' => 'LoginController@do']);
The 'as' portion of the second parameter defines the name of the route. The
first string parameter defines its route.