Multisite setup error/Class not found

Accordion to the Documentation I have to place the setup.php into the Grav root and create directories user/sites providing the necessary other directories and files.

But it doesn’t work. If I call my setup.php I get following error:

Fatal error: Uncaught Error: Class ‘Grav\Common\Filesystem\Folder’ not found in *****/grav/setup.php:14 Stack trace: #0 {main} thrown in ******/setup.php on line 14

Maybe someone can help. I think I am missing something completely obvious.

The non-multisite Grav site works correctly? Which PHP version are you using?

PHP7 the normal site is still working