Here is a guide (video instructions) on how to fix the problem that a project cannot be compiled:
"App Inventor is unable to compile this project."
Possible reason: there is a project in a project.
Caused by an extension that was imported from another AppInventor clone (here: AppyBuilder).
This issue is now also increasingly reported in App Inventor, e.g. here: My app is getting server issues and here: Unable to build apk!
See here how to fix this: