At least for me, once I got to the point you're at now, I was able to edit a subscription and add or remove a period to the end of the description or localization description. That cleared the error state and allowed me to add it to the submission. Like you said, not intuitive, but it got the job done for my submission at least.

It looks like the submission process was updated this week while my latest submission was in flight. Attaching subscriptions now occurs directly from the Subscriptions page. After it was returned for the same reason yet again today, I was able to attach it along with my build, and submit together for the first time. Hopefully this is the last step and isn't an issue for people moving forward!

Filed as FB23766490