Madden 21 can only be played on rookie mode in franchise with a friend.

@GumGumPistol407 I guess it is the same bug as has been reported by a lot of other players too (see https://answers.ea.com/t5/Bug-Reports/Madden-21-Couch-Co-op-Difficulty-Reverts-To-Rookie/td-p/9428822/page/12). I've seen there that the dev team has solved it in the December update, but since there are still reports from affected players (incl. you) afterwards, I'd guess it has not been perfectly solved. ☹️