Time Machine “Restore” button grayed out
my iMac is running very slow today, and a restart didn’t help, so I would like to restore from a Time Machine backup. I’ve found the backup I would like to use but when I try, the Restore button is grayed out. I’ve looked at earlier backups and the same thing is happening. Is there any way to restore it or are the backups all corrupted?
iMac, macOS High Sierra (10.13.6)