Lightroom Classic: Deleted photos from my computer, now I cannot remove them from my catalog

My advice is to learn some basics of lightroom. I can recomend the Free Lighroom Quickstart E-books from the Lighroom Queen: https://www.lightroomqueen.com/free-downloads/

Did you delete them in Lightroom or did you delete them using Finder/OSX or Explorer/Win? If so, then Lightroom has no idea that you deleted them and discovered they are not in the directory where they once resided. The photos should have been deleted through Lightroom; this deletes the photos and updates the Lightroom catalog. The missing photos should have an exclamation point in the thumbnails. Right click on the exclamation point and on the popup menu, there will be an option "Remove". Select this and Lightroom will give you a choice of Remove or Cancel. Select Remove and the photo will be removed from Lightroom. This will also work if you select multiple files.  

FYI -- Lr doesn't remove anything it cannot find on purpose -- a feature not a bug.  Consider cases where someone has raw/original images spread across multiple external drives.  If I'm working on stuff on externalDrive1, I connect that and work.  But I don't want it to remove everything on external drives 2-5.  

Also, "smart previews" allows me to work on images (i.e. use the Develop module) when the originals aren't available.  I cannot export or print them, but I can develop.  Useful if away from your "real" store.

Adding to Drew's note,  you can use the "Find All Missing Photos" item in the "Library" menu.  This will produce a collection in the "Catalog Panel" showing all the photos that LR Classic can no longer find (e.g. ones you deleted outside of LR using Finder or File Manager or some other tool.  You can then select one, some or all of those images and follow Drew's process to "REMOVE" (not "Delete") them.  

Dan