Add Domain button is missing on subscription if hosting is disabled in the service plan:
- Both Add Domain/Add Domain Alias buttons are missing on subscription when login in Plesk as a customer (Subscriptions > example.com > Manage in Customer Panel):
Currently, Plesk does not support adding additional domains on subscription without hosting. This is a request with ID PPPM-7124 which is planned to be fixed in future Plesk updates.
Change subscription type to "Web hosting" in order to create additional domains.