Make sure you are not opening a Lightroom 6/CC catalog in Lightroom 4/5.
With Lr closed go to the location where your catalog resides(By default it is Pictures/Lightroom)
Delete the .Lock file with the same name as your catalog.For instance in the Picture below it is Lightroom4 catalog.lrat.lock
In your case it will be Lightroomcatalog.lrcat.lock
Let us know if that helps