I have Lightroom 6 and downloaded what I thought was an update, but was probably a full programme download. On trying to open the program I got a message saying that it could not use the catalogue as it was already in use by another application. I have tried using a back-up from a few days earlier but it will not accept it. Help!
What that means is that you exited from the application and the catalog is still locked.
Go to the directory where your Catalog file is and delete the ".lrcat.lock" file, then restart with the catalog.