But I still get an error when I try to push back the PMDG 737-9ER
This doesn't have anything to do with the previous problem.
Coualt does not recognize the Parking Brake setting when I release it. Instead I get a repeated voice telling me to release the brake. So I have to abort the push back.
The PMDG 737-900 is surely supported internally, so when using the default GSX profile, the parking brake will surely be recognized. Try removing any custom profile you might have made/downloaded for the airplane, because those CAN override the parking brake test, and they might have it wrong.
You can go back to the default profile by either pressing the RESET button in the airplane configuration editor, or remove the relevant airplane folder in the %APPDATA%\GSX\Airplanes before starting the sim.