your problem is on a OSX system level. I’ve googled for similar “could not connect to CoreMIDI” errors and found some hints for fixing. Some people say they have to run MIDI Monitor or the Audio MIDI Setup utility before running the app that is trying to use CoreMIDI (in your case Dorico). You could try that.
Here is some other guy who proposes a rather brute force method: http://www.jnote.com/how-to-fix-core-midi-problems-with-osx-after-using-migration-assistant-or-upgrading-system/
But use at your own risk, don’t blame me if it doesn’t work or causes other side effects.
Normally this MIDI stuff should just work on OSX. Personally, I never had any issues with MIDI drivers on my MacMini here.
If you launch the Audio MIDI Setup utility, what MIDI devices get shown with you? If it doesn’t appear automatically, choose from the menu Window > Show MIDI Studio. Maybe you could post a screenshot.