Mounting bracket problem- installing light fixture

[Molly MacGregor]

Flipping a two bedroom condo on Cape Cod. I went to install a new ceiling light fixture but could not figure out where on the electrical box to screw in the mounting bracket. I see where it would fit, but there is just a void- nothing for the screw threads to hold onto. What am I missing? Do I have to replace the box or find a longer strip/ mounting bracket and secure that to the ceiling? It’s a very light weight fixture but that’s doesn’t sound like the solution. Is there something like a putty I can fill the voids with then screw into?