Hey folks, new HA user here.
I invested in a “Home Assistant Green” unit with SkyConnect, and was struggling with the same error as OP.
After some troubleshooting/reading, I finally got it working (as per some suggestions in responses to this thread too).
Another helpful answer that pointed me in the right direction is here
Anyways, to cut to the chase I did 2 things.
Firstly I used the device-id path as suggested at the very top of this page.
That didn’t solve it though.
Then I added adapter: ezsp to the serial config, and that did the job 
I’m now in business, the Zigbee2MQTT add-on has successfully started after that.