File not Found means you site is not finding index.php on first load.

For Laravel applications to be publicly accessible , the directory /public is responsible for making Laravel application to be public accessible.

When you commission an application(Laravel or pure PHP) via Forge and click on ‘Advanced’ tab, the web directory is set by default to /public. This is okay when you launch a Laravel app but if you want to launch a pure PHP application, you have to delete ‘public’ and just leave the field ‘web directory’ to empty