Nest learning thermostat 3rd gen - E80 error

Happy new year! I was installing the Nest learning thermostat (3rd gen) and run into a "no power on Y1 wire" problem…it is saying no power on yellow wire, NEST error code is E80. It is winter I am located in NY, I am pretty sure the AC was working in summer if they are related… Any idea where could it be wrong on this one?

Here is my situation, I have 3 nest stat for 3 floors：

Floor 1: Nest thermostat E, no C wire installed, working fine so far..

Floor 2: Nest learning thermostat 3rd gen, no C wire, reporting E80 error – no power on yellow wire.

Floor 3: Nest thermostat E, didn’t work without C wire, working fine so far after connected C wire.

I was thinking to put the "E" version device on the 2nd floor base to see if it reports same error but realized the E version’s size is different with the learning 3rd gen version so can’t do that…

Related question: Nest thermostat wiring to First Co air handler