In the case you can’t sumbit data in the popup, because it asks about some field to be filled in. But all fields are already filled for the popup.
Why this happns?
— Most likely in your Subscription service you have some fields, which are markerd as required. but you don’t configure them in the PopUp. In order to fix this, you need to find your subscription service here, find point Custom Fields and set up your custom fields correctly.