Add Domain button is missing on subscription if hosting is disabled in service plan

Symptoms

• 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):
  
• It's not possible to move the domain to the subscription without hosting, subscription is not in the list of 'Available subscriptions' in Domains > example.com > Move Domain

Cause

Plesk Obsidian versions below 18.0.80 do not support adding additional domains on subscription without hosting.

Resolution

Update Plesk to the latest version.