What is the compile error? "The compiler output error was"

I'm having trouble making my design into an app. I used the ai2 companion to connect, and tried it out, which worked fine. When I tried Build > App(provide QRcode for .apk), this error message pops up.

App inventor is unable to compile this project.
The compile error was

The problem is, it never tells me what the compile error is. It's such a simple app, so what could possibly go wrong? Any help would be appreciated.

welcome toi the community...
before asking a question, the first thing you should do is to search the community...
your question has been asked a lot...
see here https://community.appinventor.mit.edu/search?q=the%20compiler%20output%20error%20was


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