The following message appears if trying to sync subscription as a reseller:
Syncing the subscription with its service plan failed because some of the services or resources defined in the plan cannot be actually provisioned with the subscription. Available value indicates the actual state of a resource or a service in the system. Clicking OK will initiate setting the subscription values according to the available values.
This is Plesk bug with ID PPPM-1180 which is planned to be fixed in future Plesk updates.
As a workaround, a subscription can be synced under administrator account.