This message indicates that you have multiple Schedules or Real-Time Monitors trying to run the same Job. A Job can only be active and running once. If there is a Schedule or a Real-Time Monitor already running a Job it is not possible to have another Real-Time Monitor or Schedule run the same Job at the same time.
This error most commonly occurs when attempting to run a Scheduled execution of a Job that is already being run by a Real-Time Monitor. The Real-Time Monitor runs all the time which means that the Job being run by the Monitor is always in use. That makes it impossible to run a Schedule of the same Job at the same time.
This issue can be resolved by running the Job with only one Schedule or Real-Time Monitor at a time.
Please note that in versions older that SureSync 7, a Job was called a Relation.