Nest 3rd gen thermostat

Had my nest 3rd gen thermostat installed today.

The blue wire is not plugged into the nest which is causing the AC unit to turn off immediately after it turns on. It was not plugged into the old/original thermostat we had (it was not a smart thermostat).

I put the blue wire into “C” (common) in the nest and the AC turns on fine now, however the nest screen keeps giving an error that there is no power in that wire. Because of that power error, we cant control anything on the nest physicallnor through the app, but the AC itself actually runs now. Really confused as to what this can be and how to fix it.