The following error message could be found under Home > Resellers > Managed Server Customer during a sync attempt:
PLESK_ERROR: Synchronization Failed
Syncing the subscription with its reseller plan failed because some of the services or resources defined in the plan cannot be actually provisioned with the subscription. The 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
Insufficient IP settings in the reseller service plan.
Add available IP address to the service plan under Service Plans > Reseller Plans > Service Plan > IP Addresses, select IP and move it into the right column: