Deploying Flask App on AWS Beanstalk Errors? Internal server error
Based on your description it sounds like your code is trying to access the IntFlag class in Python 3.4. According to the docs this class was first added in Python 3.6.
It looks like the latest supported version is now 3.6 so - if you were to try again using this new solution stack - I think it should work. For reference, past versions can be found here.