The bot team is 34th in the European coefficient rankings, so it is too high for another player to take on. Once it drops out of the top 50/top 100 (I can't remember which) then it'll become available to manage.
and the everpresent Numpty confirmed my suspitions:
(Manual): "Teams that are in the top 50 of the coefficient ranking can not be taken over by a new manager. Only the last manager of this club can regain control of it."
Numpty: "They are currently #34 in Europe, which is what counts. They'll likely drop to somewhere around #45 at the end of this season.