Lightroom Classic stores photo records in its catalog, so deleting or moving files in your operating system does not automatically remove them from the Library. The catalog still references the old file locations until you update it.

To clean this up, go to the Library module, right-click the top-level folder that contained those files, and choose Synchronize Folder. Enable Remove missing photos from catalog, then run the sync. Lightroom will scan the folder, detect files that are no longer there, and remove those missing entries from the catalog.

For future moves or deletions, do the file management inside Lightroom instead of in Finder/File Explorer. If you delete from Lightroom and choose Delete from Disk, Lightroom updates both the catalog and the actual filesystem at the same time, which avoids this mismatch.