Aprilaire 500 humidifier wiring question - no "C" common on furnace
Find the other compressor wire, and that's your C

There will be three wires going to the outdoor unit in your setup -- one comes from the Y wire from the thermostat (call for compressor), another comes from the O wire from the thermostat (reversing valve), and the third and remaining wire is the return path for both those signals, so it must land on the C terminal, or whatever your system calls it (likely B, but can't tell for sure from your photo).