"realpath(): open_basedir restriction in effect. File(/) is not within the allowed path(s)" error

This error happens when PHP has its access limited by the value of open_basedir PHP setting.

open_basedir limits the files that can be accessed by PHP to the directory-tree it is specified on this setting. And setting a wrong value can be problematic with some plugins, including WP Rocket.

To fix the problem, you should set the value of this setting to " none". 

If you don't know how to do this, you can contact your hosting provider, they should be able to change this for you.