Why cant i sell

Hello @n801c65hnhmh 

The picture clearly shows that you still have your player in the line-up somewhere.

The shirt symbol on the right side of the player (Kane) shows you that🧐
As @Sanyosa has already written below, you have to make sure that the player you want to sell is no longer in any line-up.

I hope we were able to help you and that you can sell the player at a good price😇