You can try
php artisan make:controller Admin\\AdminController --resource
It will create the AdminController in Admin folder.
You can also use --plain or --resource to generate your desired Controller.
php artisan make:controller Admin\\AdminController --resource
The backslash (\) character is used to mark special characters
so that they are not interpreted by the shell. so Adding another backslash (\) will escape the second backslash (\).
In some terminals below command can also work.
php artisan make:controller Admin/AdminController --resource
You can find more about backslash (\) here
Hope this helps