It's your GPU cause this "error", which should do nothing to your airport card.
Some 2009 Mac Pro shipped with an airport card that not supported in Sierra. In your case, most likely you have that card. So, if you really need Wi-Fi, the options are...
1) downgrade back to 10.11
2) buy a new airport card that has native 10.12 support
3) buy a USB external Wifi dongle which can be used in 10.12 (3rd party driver may be required)