When you launch Lightroom Classic, you encounter the error "The Lightroom catalog cannot be opened because another application already has it opened."
Go to the folder where your catalog file [yourcatalogname].lrcat is saved. By default, Lightroom Classic saves catalogs in the following folders:
- Windows: \Users\[user name]\Pictures\Lightroom
- Mac OS: /Users/[user name]/Pictures/Lightroom
where [user name] is the user id with which you are logged in.
When Lightroom Classic is running with a catalog open, it creates a [yourcatalogname].lrcat.lock file next to the [yourcatalogname].lrcat file. This file ensures that there is no other access to the catalog in use. When you exit Lightroom Classic, the lock file is deleted automatically.
However, if Lightroom Classic is not shut down properly, the .lrcat.lock file remains in the same folder as the .lrcat file, which prevents Lightroom Classic from opening.