First time testing old motion sensor outdoor light, with new LED bulbs, light flickers at constant fast hz, why?
This is due to the way older motion sensors worked.  Incandescent lights essentially ignore (or at least, don't put out any light) from small amounts of power coming through the fixture, which is how the motion sensor is powered.  However, LED lights will light up with very small amounts of energy, so that power leakage will cause them to flicker off and on.  It's the same reason LED lights have problems with older dimmers.

Basically, you have to either install incandescent (possibly halogen) lights into the fixture, or replace the fixture so it's compatible with LED bulbs.  Or get one of the combo led panels/motion sensors.