Error message on starting Lightroom

Is the catalog folder inside a folder that syncs to the cloud, like Dropbox or OneDrive?

First check if there's a .lock file next to the LR catalog in the appropriate folder.

If  there's such a .lock file it must be deleted manually.

More informations about this, you'll find here: 

The Lightroom catalog cannot be opened because another application already has it opened - lrcat.lock (adobe.com)

If this doesn't help try resetting the preferences of Lightroom Classic:  How to reset Lightroom Classic preferences (adobe.com)

It's recommended to backup your preferences before you reset the preferences to the default settings: 

https://helpx.adobe.com/lightroom-classic/kb/preference-file-and-other-file-locations.html

If this also doesn't help restore the catalog from the recent backup. If you don't have a backup you can also try to repair the existing catlog or create a new one and then check if it's works. To do that hold the Ctrl-Key while starting LrC. This opens a catalog selection dialog.

First select the appropriate recent catalog and let it check it – note the checkmark "Test integrity of this catalog" to check its integrity. If that doesn't help, try to create a new one and see if this catalog works. If yes, you could then try to import your current catalog into the new one.