If your writing device works in the tutorial, but not in the actual score editor, it is likely because you are not using an official Apple Pencil.
StaffPad for iPad requires an official Apple Pencil (made by Apple themselves) to work properly. This is because StaffPad relies on many native features of the official Apple Pencil, like pressure-erase, for instance.
If you have bought StaffPad and don't have an official Apple Pencil, you can purchase one here.
You can also request a refund for StaffPad through the Apple Support system. More information on that process can be found here.