DIAGNOSIS The E1 error means the thermostat isn't detecting a connected 'W' (heat) wire. This usually happens after installation or if a wire came loose.
FIX Turn off the power to your furnace at the breaker. Pull the thermostat off its wall plate. Look at the terminal labeled 'W'. Is there a wire (usually white) inserted into it? If the slot is empty, you need to connect your system's heat wire there. If there IS a wire, make sure it's pushed in all the way and the terminal screw is snug (don't overtighten). Snap the thermostat back on and restore power. The error should clear.
IF THAT DIDN'T WORK If the wire is secure at the thermostat, the other end might be loose at the furnace. You'll need to check the furnace control board. With the power OFF, locate the same color wire coming from the thermostat bundle connected to the 'W' terminal on the board. Ensure it's secure there as well.
HEADS UP If you never had a wire in 'W' because you have a heat pump system (which uses 'O/B' and 'Y' for heat), you may have told the thermostat you have conventional heating during setup. You'll need to go into the installer settings in the app and change the equipment type to "Heat Pump." That will make the E1 error go away because it will stop looking for a 'W' wire.