Cause
An attempt is being made to sync a shardgroup where the replication unit specified has readwrite status without the -syncAll flag specifed. To sync a shardgroup with a readwrite leader for that replication unit, -syncAll flag must be specifed, which will sync all shardgroups in the shardspace for the specfied replication unit or units.
Action
Retry GDSCTL 'sync ru' with -syncAll parameter to sync all shardgroups.