Lightroom 6 won't open Mac 10.15.5

As the install of LR v6 is a lost cause, moving on to

so subscribed to LrC and Lightroom CC, installed them both but my original catalogues won't open in them. 

Why not? What occurs when you launch a old catalog?

First up, locate using Finder, your catalog. As you had some issue, check for a lock file and deal with that. Then double click on your catalog.

What should happen, is LrC should launch, it should note that your catalog is in need of an update, you select yes, LrC copy's the old catalog, gives it a slightly new file name (adds a suffix) and opens the new catalog, this is required as the database engine is different.

about lock files:

https://helpx.adobe.com/lightroom-classic/kb/lightroom-error-catalog-cannot-be-opened-lrcat-lock.html

be very very careful.

about Lightroom Classic upgrades causing catalog updates:

https://www.lightroomqueen.com/lightroom-upgrade-catalog/

As you've been told, you cannot install LR6 under Catalina. But if you install it with an older OS, it will run (within reason, it's unsupported). I'm running an old copy of LR5 under Catalina (Maps is busted but otherwise, seems OK). IOW, LR5 and LR6 can run under Catalina if installed prior to the upgrade. 

Hi there,

We're sorry about the trouble with Lightroom on your Mac. Lightroom 6 and older versions are not compatible with macOS Catalina. Please check: https://helpx.adobe.com/lightroom-classic/kb/macos-catalina-compatibility.html#ArelegacyorperpetualversionsofLightroomClassiccompatiblewithmacOSCatalina

You can try opening the Lightroom 6 catalog with the latest version of Lightroom Classic and it should prompt you to upgrade the catalog. Once the catalog upgrade is complete, you should be able to access the images and edits from the catalog.

If you are facing issues doing that or if you are getting an error, please share more details (possibly a screenshot of the error) and we would be happy to help troubleshoot the issue.

Regards,

Nikunj

You can't re-install Lightyroom 6 on OS X 10.15 or later. Apple does not allow the installer to run, as the installer is a 32 bit program.

When you say LrC won't open the LR 6 catalog, what happens? What is the exact word-for-word error message?