Could not open input file: artisan
You need to first create Laravel project and if you already have one you need to go to this project dir using cd command in terminal for example cd myproject.
Now you will be able to run any artisan commands, for example running php artisan will display you list of available commands.
You cannot use php artisan if you are not inside a laravel project folder.
That is why it says 'Could not open input file - artisan'.
If you project is at /home/forge/laravel-project/
You can properly execute your artisan like this
php /home/forge/laravel-project/artisan ...