OK I have worked out what the issue is. I only use this server for DNS management (Hosting Type for all subscriptions is "No web hosting" or "forwarding" and it appears that you can only add a subdomain where a subscription / domain has hosting. I have a different server for the very small...
When I try to add a Subdomain to my Plesk system ( Plesk Version 18.0.58 Update #2) I am getting the error "Warning: Unable to create a subdomain: There are no domains available"
when I click on "Add Subdomain" in the domains screen.
In the Service Plan for the domain I am trying to add the...