My tradeable player became untradeable

I just got the anniversary mbappe 113 ovr from the market about a week ago , after some matches I get on the market to compare prices (was trying to decide if I should still keep him) but the transfer section on him is locked (basically saying he’s untradeable). I want to know how this is possible and what can be done to fix it. I spent about 3.5 billion for him.