its been awhile but i think this is how it goes.
1 either you need to go to franchise options and turn resign players to user control
2 most likely problem is when you see the list of offseason tasks, you have to go into resign players. resign whoever you want, then click continue, start, whatever... simulate from this screen. if you back out of the resign players screen, and go to the tasks screen and simulate, then the computer will simulate the resigning. so make sure you go into resign players, then continue. dont simulate from the offseason tasks screen. i hope you understand all that i suck at explaining stuff.