Remove the back plate (the gold dome that can't be tightly secured) and check to see if you can turn the stud (the screw that the nut goes on) in a clockwise direction.

If you can do this then you will not have to undo anything else.

If the stud cannot be screwed deeper into the electrical box then you will have to shorten it and this will probably involve taking out the fixture bar.

Be sure to cut it with a screw cutter to maintain the integrity of the threads.

If you make a mistake take the screw to a hardware store for a perfect match, but you probably will need an 8x32.

^ the screw being described would be the one in your crossbar bracket, that you would back out so there is less sticking out from the bracket that your nut attaches to.

This is a common issue with this type of mounting. You've gotten good advice here, but don't be surprised if you need to make adjustments and trial fits 2-3 times before you get the bolt length just right.