I am using a jailbroken 4th Generation iPod Touch running 6.1.3. I have installed a jailbreak tweak to allow me to send messages via google voice called SMS on GV. I set it up, went into BiteSMS to send a message and I got a message saying "Cannot Send Message: iMessage needs to be enabled to send this message." (the same happens from the messages app) When going into Settings>Messages and turn iMessage on, it will turn back off. This also happened before installing the tweak so I don't think it has to do with the tweak.