Hello Vermans.
Watch out for the following issues if you find the AAPT error.
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Project naming. Make sure your project name starts with an alphabet, and only contains English letters, numbers and underscores(_). Ref: Help build error (AAPT execution failed) - #11 by TIMAI2
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Check out for this list of potential issues: ProfessorCad: Tips & Tricks
These are a list of topics with the AAPT error: Search results for 'AAPT error' - MIT App Inventor Community. Please search the community next time for similar queries; this allows you to do research yourself.
If the issue persists, post the AIA file of your project here.