I own LR6 on MAC and I downloaded the trial version of LRC to understand what advantages it has. After a few days I tried to open LR6 again but it doesn't open anymore. I tried starting from an old backup catalog, I deleted the preferences, but nothing, it doesn't open anymore. The image of the program being loaded appears for a moment and then disappears immediately. Thanks for your help.
I can't be sure, but my guess is this: If you tested on MacOS Catalina or higher, then the test may have caused your license to become reset to a subscription. To correct this Lightroom 6 would want to re-register, but the registration part is 32 bits and so it does not run on these 64 bits only systems.