Installing Nest 3rd generation thermostat from Old Trane Weathertron Thermostat (Mercury one)

While you do not state if this is a heat pump unit - the wiring indicates it is..

Your NEST screen shows that Rc (24Volt power for Cooling Circuit) is connected and Rh (24Volt power for Heating Circuit) is not connected, yet in your picture for wiring hook up your red wire is connected to the Rh line (so how does Rc get power and show a connection?). The Nest is either reporting wrong - or you have changed something between pictures.

You can run a jumper across from Rc to Rh since you only have one Red wire. Unless your NEST configuration allows you to to do that through software - if the software does this for you when you select heat pump - I would venture the red they will have you connect to Rc and program jumper to Rh it might aut connect it as well.

You will also need to make sure you specify in the NEST system that you have a HEAT pump unit - this might be a reason Rh does not work and Rc does.

Why you have BLACK X2 to HUM (which I think is humidifier) - I don't know. X2 is supposed to be AUXilliary Heat and would be connected to W2.

You can see my post here for Thermostat wiring proper color code and also what the individual labels mean - what circuit they are for.

T (brown) wire is the common on my trane weather tron. The blue wire IS NOT used on the Nest. And x2 calls for emergency heat on the t stat.