There are wood laths all across the ceiling. The trick is to screw through the white plaster hopefully into a wood lath behind. If the screw doesn't get a fixing, you move the metal strip and try again! By changing the angle of the bracket compared to where I've drawn it, you should be able to screw into the wood you see.
If the light is heavy, you need to find a joist to screw into.
If the light is heavy, you need to find a joist to screw into.