I was in the same predicament as you. I use LR as a hobby/stock photographer. I shoot a lot with my mobile phone and little with a camera. Recently I need to clean up my hard drive and wound up removing and reinstalling LR on my laptop. When I tried to sink with my mobile device it was giving me the same response. I searched for an answer and didn't find it. So I decided to go out on a limb. I keep all of my original work backed up to Google Photos. So if I lost all the work I wouldn't lose all the photos. Deleting your previously synced catalog will delete all of your work. I'll repeat DELETING YOUR PREVIOUSLY SYNCED CATALOG WILL DELETE ALL OF YOUR SAVED WORK. So that being said I would find some way to back up your work. You may be able to back up your catalog, delete the previously synced. And then restore the catalog. I suggest getting in a chat with Adobe and trying to work through. For me this wasn't that big of a hit as all my work is already backed up and saved elsewhere. I will just be starting my catalog over. Hope this helps.
As you can only sync one catalog to the cloud your solution would be to use just one catalog. In this case I will assume that the iMac is the working machine upon which you will do your edits. So the catalog on the iMac is the one which will be synced.
If you have started a catalog on the iMac:
A suitable method here would be to transfer the catalog from the Laptop to the iMac. (Copy the .lrcat and associated preview files)
Open this catalog and then from the menu select File > import from another catalog. Navigate to the catalog you setup on the iMac.
After this import has completed you can delete the iMac catalog file and you will be left with a merged catalog bearing the same name as that on the Laptop. This will already be synced and should contain your synced collections. If those collections are intact go ahead and start the sync process from the Name Plate drop down menu.
The catalog on the laptop should no longer be synced - IE. turn that off.
If you have not started a catalog on the iMac then copying the catalog from the Laptop is all you need to do. Then start syncing from the iMac and turn it off on the Laptop.