The lock file is in the same folder as the catalog. The catalog is ususally in a folder called Lightroom Catalog inside the Pictures or My pictures folder, unless you decided to save the catalog elsewhere.
You can also use the operating system to search for .lock files. Make sure that the OS is set up to show file extensions.
Sorry, I should have been clearer. I know where to look for .lock files, and have searched both manually and using the OS search option. However, there are no .lock files showing up on my computer. Only the single .lrcat file is showing. I've got hidden files showing.
Is there another issue apart from the .lrcat.lock issue that can cause this error message? If not, why are there no .lrcat.lock files showing on anywhere on my hard drive?
I have never encountered a case where there is no lock file present with that error message.
Have you tried restarting your computer?
Yes, I tried that as a matter of course, but sadly it did not work.
What has worked though was copy-pasting the catalog together with the Smart Previews in to a different location. They now open fine from that new folder.
I know this is perhaps maybe a dumb question. But do you have your system browser configured to show file extensions?