Lightroom 6 catalogs are not backward compatible with previous versions of Lightroom. The database engine is upgraded every time there is a major upgrade of Lightroom. The catalog is automatically upgraded to the new version, but the new version cannot be downgraded back to the previous older version.
You have upgraded to Lightroom CC/6 on your system and that has converted the Lightroom 5 catalog to a Lightroom CC/6 catalog. Which can only be opened, accessed, from Lightroom CC/6. It is not backwards compatible.
I understand that. I'm trying to open via my newly installed Lightroom 6 application. I, also, attempted to open by clicking on the new Lightroom 6 catalog file itself. Same error message.
I'm not clicking on Lightroom 5. I backed up that catalog and left the application on my Mac for now, but I am not attempting to use it.
If you open Lightroom 6, and then go to Help > About Lightroom, which version does it say?
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So it seems that LR 5 is opening when you click the LR 6 shortcut.
I don't use a Mac, so somebody who does might be able to give you better advice, but I would first try to uninstall LR 5.
If that doesn't help, uninstall LR 6 as well. Then reinstall LR 6.
If that doesn't help either, uninstall both programs, then run the Adobe Cleaner Tool followed by a reinstall of LR 6.
Uninstalling will not affect the catalogs or your photos.
Before I saw this reply from Per Bernsten I cleared out then reinstalled LR 6 and that seems to have done it. I didn't know that uninstalling wouldn't affect catalogs, so thanks for that. Will be good to know in the future! I'm nervous whether I've truly fixed my LR 6 problem or not, but fingers crossed.