The H2 error is a result of the Mysa brand thermostat sensing an open heating circuit. To complete the heater circuit:
If these connections are good, please test the heater circuit with a digital ohm meter.
Here is the guide for single conductor heating systems: https://www.warmlyyours.com/en-US/publications/OHM-READING-30032-A
Here is the guide for twin conductor systems: https://www.warmlyyours.com/en-US/publications/TZ-TWIN-OHM-READING-30033-A