It can happen that a cluster becomes invalid.
A printer can have newly installed printer software.
A printer is no longer registered.
The number of cluster printers is fewer than two or more than 100.
What to do
Read the message.
Hover over the 
symbols of the printers to read more information.
Remove the printer with newly installed software from the cluster. In the cloud service administration portal, first remove the printer and then register the printer again.
Remove the non-registered printer from the cluster. In the cloud service administration portal, register the printer again.
When you have solved the encountered problems, start the synchronization again.