As we near the end of the term, lots of grade syncing is happening and we've seen quite a few tickets come in for an error message that pops up when attempting to sync grades from Canvas to PowerSchool.
This error message simply means that there is already a grade sync in progress for the course. Since you can only have one grade sync running at a time, it errors out.
Canvas is working to update the wording on this error message for clarity, but in the meantime, you can check the status of any current syncs by going to Grade Sync > History.
If you notice any discrepancies between Canvas and PowerSchool, try running a Remaster Grade Passback from the Grade Sync > Settings page. This process takes a little bit longer to run but will sync assignments more than 30 days old as well as fix most sync errors.