www.makemkv.com

Niddik I have seen that error on Mac on and off for years and across many, many versions. Are you using a USB powered drive by any chance? I think, in my case at least, it is a drive/Mac issue rather than MakeMKV and it is the Mac no longer seeing the drive (I couldn't view the content in Finder eit...

If you just double click MakeMKV under Catalina you can't run it, you just get Cancel and Move to Bin (or Trash depending where you are). Either follow Apple's convoluted instructions linked above or just Option-Click the MakeMKV app in the Applications folder. You will still get the gatekeeper warn...

On my old 2012 MBP it sometimes doesn't see the drive if I insert the disc before starting MakeMKV. Eject the disc, restart MakeMKV, reinsert disc and it works. May be the same with external drives on newer Macs?