With many Aprilaire thermostats, the flashing 11 indicates that there is a broken wire between the thermostat and the HVAC system. You should turn off the power to the HVAC system. Then take the thermostat off the faceplate. Check for any damaged wires. If there is a break near the thermostat, take the undamaged section of wire and secure it in the correct spot. See https://www.manualslib.com/manual/8847/Aprilaire-8346.html
for the installation directions.
You could also try installing a new thermostat. Then make sure that all of the wires are properly connected to the terminals. You may need to check the wires at the HVAC control board or in your wall. You may want a professional to check the system for either of these steps.
I hope this helps.