the lightroom catalog is corrupt and cannot be used or backed up until it is repaired.
The repair operation may take several minutes to complete and should not be interrumped while in progress.
A messange with further instruccions will be displayed when the repair process is complete.
So what happens when you try to repair?
Try opening the most recent backup of the catalog. LR makes a backup of the catalog once a week by default. So if you use LR often your most recent backup is only 6 or less days old.
Retrieving data ...