Your Namor (or Cull for others) might be in a currently running Quest or AW (or AQ later on for others when AQ starts and if they have similar issue).Finish the Quest, or wait for AW/AQ to end.Remember, after selling, you won’t get them back immediately, it will take many weeks (sometime in March) before they will get returned to you at Rank 1.
Thanks , kabam ! I did it)
You are not able to sell five star or six star champions in mcoc.