Lightroom catalogue problème

I agree with @AxelMatt : You are trying to 'open' a  Version-12 catalog with Version-11 of Lightroom-Classic.  LrC Catalogs are not backwards compatible. ie. v11 cannot open v12 catalogs.

Also Lightroom v6.1 has no part in this problem! v6.1 is the Cloud version of the "Lightroom" apps- NOT related to Lightroom-Classic at all. COMPARE LIGHTROOM & LIGHTROOM-CLASSIC FEATURES 

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 license for 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. 

You can download the installer application file for v6.14 here.- Lightroom 6 End of Support 

I'll recommend saving the installation file for further usage. Because it isn't sure how long this download will be available.