I had this happen not to long ago. My issue was, I didn't have enough money so the OC made more than my position players. Thus, the OC would not accept the position.
I waited until the 1st pre-season game and a OC was selected for me and the message about "coaches salaries are out of whack" appeared. I was allowed to adjust this. After that happens, you can switch out the OC if you don't like the one chosen.