Request for code review - property based test

This is my first time writing a property test. I hope to know if the test has some bad practices.

The second module is a Shim. In my case, I would like to test a different scenario, such as create a user who already exists in the system. By using the shim, I have :create and :create_existing to indicate different scenarios. So, if calling :create_existing, an error is expecting instead of success.