Need some assistance on replacing a light fixture
First of all make a couple of measurements.


Check the distance between the long screws as you have them set right now and compare to the distance between the key hole slots on the fixture base. They need to be the same.
Measure the depth from the panel that the keyhole slots are in to the top most rim of the fixture base. This distance needs to be a little less than the long screw head protrusion below the ceiling surface. The installation guide says that the protrusion needs to be about 1.2 inches. If screws are too short maybe you pre-installed them into the box holes too far .. or you will have to acquire longer screws.


Make sure to connect the GND wire from the fixture base to the bare wire safety ground wires in the electrical box.

The cross bar installation looks OK but it is not doing much for you.