Disabling ini_set() is not uncommon on some shared servers, but not truly necessary in kiwitrees. It is used in various places to enable admins (only) to adjust things like php memory and time-out settings among others. This error usually appears at the final step of the kiwitrees setup process.

If you web host provides a management tool such as cPanel you can enable functions like this.

Look where you set or change the version of PHP (PHP -> Select version) and then under version what plugins to enable. Then Under “Options” find “disable functions”. Remove ini_set from that disabled functions list.