I am wondering the same thing. We have a SPED behavior app purchased and it has modules to be added as ' in app purchases' but it says it cannot allow a subscription to be purchased since there is no Apple ID associated with the app.
This issue has not been resolved. I just had the same issue come up with the Capture One - Capture Pilot iOS app that prompts for an In-app purchase after installing. I followed up with Apple Enterprise Support and found that the only way to do in-app purchases is to remove the "purchased" app via Workspace One and re-install it locally after creating an Apple ID and logging into the device. Then you can pay for the in-app purchase by associating a credit card with the Apple ID or purchasing Apple Gift Cards and associating with the Apple ID.