Please move the copy of the backup catalog onto your internal drive and then try it again.
It is possible that Lightroom needs to create a temporary copy because your version of Lightroom is newer and it needs to upgrade it.
Once Lightroom is opened, you might have to re-connect the images.
Hope this helps.
I don't understand. Both the Mac Book and iMac I have are running LR6. What do you mean "move the copy of the backup catalog onto your internal drive and then try it again"? The backup catalog is on the external hard drive, it as a copy of the catalog on the internal hard drive of the Macbook. I am trying to import the Macbook catalog from the external hard drive to the IMac.
There is a good chance that the problem is a permissions problem on the external drive since the files there were created by the backup software. Copying the catalog from the external drive to the internal drive would remove the permission problems from the processing.
Joseph has provided you with correct information. I would request you to follow the steps.
Let us know if it helps.