I find if I lift the deck all the way to the top and then turn the knob, it does not bind, if I do not , then I can not turn it. Try raising the deck via the 3 pt control and see if that helps.
Lift the 3 point hitch (and mower) to the top, then adjust the height adjustment knob. I believe that’s the way it should work. If not you have a problem.
I had the same problem. My was caused by the shaft binding on the sheetmental where it passes through. The hole was not centered, so I filed the hole a little oblong. Works fine now.