No C for new thermostat
It's all in a name

The silly folks who made your air handler decided to call their C terminal a B terminal instead, perhaps because it has a blue wire hooked up to it in the wiring diagram.  This blue wire goes to CN-1, which connects to the blue wire (return) from the fan relay and to the blue/black return wire to the transformer.

So, hook your thermostat C wire up to the B terminal and call it a day.