steering coupler

Had the same problem with a Cardone box. Inspect the splines on both very carefully. I finally got it on but it turned out to be a really sloppy piece of crap! My alignment guy thought it was a worn out original I had cleaned up , 10° of play in either direction.
Replaced it with a Firm Feel Stage III , coupler slipped right on with no effort.

Hope you have better luck with your box.