It seems your try to open a catalog which generated/updated with Lightroom Classic 12 in an older Lightroom version.
Search in Finder for .lrcat - files. If your catalog was updated there is a copy of the existing catalog.
Which version of LR would be open when you start the program?
You wrote you have installed LR 6.1. But the screenshot is from a newer version.
Do you have installed a update to your existing Lightroom or installed a trial verion of a newer LR?
I'm afraid you have 'upgraded' a perpetual ('Unbegrenzt') license of Lightroom 6 to Lightroom Classic somehow. That upgrade should never have been possible, because you don't have a subscription to Lightroom Classic, but a perpetual licensefor Lightroom 6.There is no perpetual license for Lightroom Classic.
The solution is to go back to Lightroom 6, the version you have a license to use.