In case someone else is still trying to use this on Catalina and getting the "MIDI engine" error, I was able to run the Eleven Rack Editor after the following steps:
- Disconnect the USB
- Open Audio MIDI Setup (Cmd-Space > "Audio MIDI Setup")
- Window > Show MIDI Studio
- Select and remove the greyed-out eleven rack MIDI device
- UNINSTALL the Eleven Rack drivers
- Reboot
- Connect the Eleven Rack via USB, which will install the MIDI device properly
- Eleven Rack Editor should now run properly
- Reinstall the 11R drivers (if you want to use it as an interface)
- Reboot