pi-topOS is designed to be closely related to Raspbian, to ensure compatibility with tutorials across the internet. However, in some cases, the modifications and changes that are used in pi-topOS mean that these are not possible.
See What are the main things that are different in pi-topOS? for a more complete list of modifications that are applied to Raspbian.
If you are following a tutorial, check that it is for the latest version of Raspbian/pi-topOS (currently Buster - previous versions were called Stretch, Jessie, Wheezy...). Tutorials that are a couple of years old may now be out of date if they are trying to make changes to system-level files.