WTF do i do ??

here's a stupid question for you:
how the hell do i trade players ?????

the "EXECUTE TRADE" option in 'trade players' mode is blurred and disabled, and i just have no idea what to do in order to trade players.

PLEASE HELP !!

Drag the players you wish to trade to the box below each team's roster. You can then click the "Execute Trade" button.