I have been using Lightroom Classic for years and have ended up with a few folders in LR (version 12) that have a question mark on them. They no longer exist on any hard drive on my computer, thus they have no images in them. I know that I should only perform file/folder manipultation via LR, but over the years with a large catalog occassionally I have made changes outside of LR to the folder structure, rarely though. Anyway, when I right-click on the folder and choose "remove", nothing happens. And if I try and create a blank folder on the hard drive and then point "Find Missing Folder" to that new folder, again, nothing happens. I cannot seem to get ride of these orphaned folders! I have tried testing the integrity of the catalog upon Lightroom startup. Advice?
If resetting the preferences does not work, then the catalog may be corrupted. You can (probably) solve this in the following way. Create a brand new catalog and then choose 'File - Import from another catalog'. Select your old catalog. Import all photos without moving them. Because the missing folders in the corrupted catalog do not contain any images, they won't come across. Everything else will, except any publishing services you may have.