Nest thermostats are designed to use very little power, and in many cases they can use your system’s heating and cooling wires to get enough consistent power. But sometimes, you may need to connect a common wire (C wire) or power accessory, and the app is telling you this is the case to make your system compatible with Nest Thermostat.
It sounds like you've already checked, but look at which wires are connected on your system’s control board to see if any of them are connected to the system’s C terminal. If so, you can put the other end of that wire in your Nest thermostat’s C connector.
If there isn’t a wire in the system board’s C terminal, you need to install the Nest Power Connector or have a local HVAC professional, install the power connector or a C wire for you.
