Unless incorrect brackets, i don't see anything wrong with the code.
You may need to remove unnecessary brackets.
Code:
on *:text:!start:#: {
  if ($nick isop #) { 
    msg # /me AutoCom: ON 
    timerTwitter 0 600 msg # Twitter Link! 
    timerDonate 0 900 msg # Donate Link 
    timerGT 0 300 msg # Gamertag Link   
  }
  else { msg # /me $nick -> You are not allowed to use this command. }
}

on *:text:!stop:#: {
  if ($nick isop #) { 
    msg # /me AutoCom: OFF 
    timerTwitter off | timerDonate off | timerGT off  
  }
  else { msg # /me $nick -> You are not allowed to use this command. }
}