How do I fix this error notice when I try to open Lightroom, it says it cannot open because it is open in another catalogue, I don't have another catalogue.
Using your system browser, open the folder containing your catalog. You should find a file with the .lock Extension. It should have the same filename as the catalog. You need to delete that lock file. Sometimes if Lightroom doesn't shut down exactly right that file gets left behind. It's a file that is used by the catalog, but is deleted whenever Lightroom closes.