The title of your post is confusing. Did this happen after upgrading your CC 2015 catalog to Classic?
Are you sure that you're looking in the correct folder? Maybe your're looking in the CC 2015 catalog folder?
Also, you need to have file extensions enabled to see the .lock extension. I'm not a Mac person, but instructions are here:
Use your operating system's search feature to search for all files whose name ends with .lock
Delete the .lock file found in the folder which also contains your catalog file. Do not delete the catalog file itself, just the .lock file.
I like to say it in detail:
1. I used Lr 2015 so far
2. I installed Lr Classic CC (because Lr2015 coult not import the RAWs of my new Camera "Sony Alpha 9"
3. As I tried to start with Lr Classic CC – I got the Massage:
“Der Lightroom-Katalog “Lightroom Catalog” kann nicht geöffnet werden, da er bereits von einer anderen Anwendung geöffnet wurde.“
Means, as I anderstand it “the catalog can not be opend because it is used from other application”
4. I found the Information from adobe: I have to remove the file: [Lightroom].Ircat.lock
5. But on my iMAC I did not found such a file, even not with the search for “.lock”