Subscription is restored from other Plesk server, where subscription has limit maximum websites=1.
While creating the second domain on subscription, the following error appears:
Error: "Warning: There are no available resources of this type (sites) left. Requested: 1; available: 0."
There is no button Add domain in the subrscription.
It appears only if you restore subscription with website limits=1 (you can check it in subscription properties). Subscription properties/limit are restored as well during domain restorationl. Software issue #PPPM-1948 was submited for this case.
To allow to create additional domains under subscription, increase number of sites in subscription using command line:
# /usr/local/psa/bin/subscription_settings --update domain.tld -max_site -1
Or you can do it via Plesk GUI > Subscriptions > Domain.tld > Properties .