The H2 Error means the thermostat did not detect a heater connected Mysa's load wire. If you see this error, you'll have to reinstall your Mysa with a different wiring configuration. If you need help determining your specific wiring configuration, consult this guide.
If you're sure the thermostat is correctly wired, it's possible there's a poor connection between Mysa's load wire and the heater's wire in the wire nut itself. If this is the case, remove Mysa, and make the wire nut connection again. Follow this guide to learn how to correctly using a wire nut.
If you still need help, read about contacting support, here.