Most commonly, this error implies there are two cases for one client. One that was pulled from Clio and one that someone in your firm created manually using the add case button. Cases added manually do not sync back to Clio since there is likely already one record which is syncing for that particular case.

There isn't a way to merge cases but you can export the messages from the case which is not syncing, and use the export to update the case in Clio. For convenience we have created a video explaining how: https://youtu.be/jNmNq1zGgbU

Once this process has been completed you can just remove the case case which was not synced to Clio by clicking the "Remove without Penalty" button found at the bottom left hand corner of the Case Info screen.