Try inserting the above header manually, in my case it fixed the problem. It is possible that this is specific to the ArchiCAD 26 API, by the way, missing something here is a fairly common situation that many of us face.

Help will probably help, although such basics are best acquired on your own. Of course, at the beginning you will face problems like these, but in time their solution is not as complicated as we think, but such guidance is worth appreciating.