Make sure a viable battery is installed in the bike. If you havent tested battery voltage yet, do so first to ensure you have a working battery (SOC lights illuminating are not enough to ensure the battery works).
Also make sure that the voltage is present at the power plugs under the motor cover before you test the display harness. If you have already done so, test the main harness as follows:
Unplug the display from the main harness.
On the plug coming from the main harness read voltage across the following pins.
You should get full battery voltage across those two pins if you have a working motor controller and main harness. If you do not get any voltage either the main harness is unplugged or the motor controller has failed.