Lightroom 6.4 is not compatible on Mac 10.8.5 : System requirements for Photoshop Lightroom for Mac and Windows OS
You need to either upgrade your operating system or revert back the update for Lightroom.
Thank you for your reply.
In that case, I would like to revert back the update.
How is that done?
But if I uninstall lightroom, won't I lose the years of work I have done up until now?
I'm afraid I'm not very tech-savvy.
No, you will not loose your work as you would uninstall only the program.
All your is in your catalog which would remain as it is.
Like I say... not tech savvy. I thought my catalogue was in Lightroom. Where is it?
Is that the same as Finder? I thought that Lightroom itself contained all the edits that I have done to my thousands of photos.
I'm not sure I have the confidence to do this myself.
Lightroom creates the catalog in .lrcat format and it does not get affected when you reinstall the program.
When you import image sin Lightroom, it does not keep the original images in it. It always references to the original images from where they were imported and Lightroom remembers that location.
Due to this reason, all the edits that you do in Lightroom remain only within the catalog unless and until you export them.
You just do not have to touch catalog at the time of reinstalling.
If you want, you can copy .lrcat file which is the catalog and paste it at any other location on your machine so that you will have a backup copy of your catalog.
Thank you for your time and for your help.
I will try this.
Let us know if it works well once you have done the steps.