Installing a Ring Flood Light Camera
The type of "garage motion light" that most people are familiar with has a motion detector and two floodlights built into one package.  This is what the Ring is trying to duplicate.  However, there is a more... architectural option.  You put only the motion detection pod where its needed, and it controls other lights that are mounted separately.

I believe this second case is what we are talking about, and in this method, the motion switch just acts like a light switch and doesn't have power going to it.  If there are only two wires in the motion detector box, then this is acting like a switch loop and you can't install a Ring light there as it sits.

If conduit is running to the box, you may be able to pull an additional wire, but there is another option.  If you don't want to switch the existing lights any more (they would be dead) and instead want only the ring light to work, you can turn the switch loop into a normal hot and neutral wire.

To do that, you would want to open up the boxes for the switched lights on the garage and see where the white and grey wire to the motion detector end up.  Once you find the other end of the white and grey, you can then rewire them to provide power to the Ring.  If you need more help with that, it would be a new question with pictures, but that's the basic idea.