Lightroom Classic CC 7.4 synchronize folder function greyed out

'Synchronize folder' is an import method. It's a shortcut to the import dialog, mainly for when you have new images in a folder that Lightroom already knows. It is not some magical way of solving all problems that people can encounter. If you have a problem with a file that is already imported, then 'Synchronize folder' is not the way to try to solve it. It won't do anything because there is nothing to import.

If you can't get a viable preview of a file, then there something wrong with that file. Many things can happen, so you need to give us more information. What version of Lightroom? Which OS? Which version of the OS? What happens exactly? Do you see an error message of some kind? Did the file ever have a good preview or is this a problem that started right after you imported it? What about a screenshot to show us the problem?

Have you moved folders with the OS ?

I thought that may have happened as you talking about a disk defrag . . .

Also it has been noted that capitalisation of either the path/folder name could be problematic from time to time.

Any chance of a cropped screen shot?