Hi and welcome to the forum!
I just did the update 2 days ago and it is a large one. It kept updating, than waiting and went thru this cycle about 5 times before it finished. At one point I restarted my iPad thinking that the update was in a hang phase, but it kept going.
If it is saying that it cannot update, you may want to restart the device to see if that helps. Once again, it is a large update. You can press the app to "pause,"but once you press again it will start again. It is not playable until the update is complete.
Try not to use any other apps or go on web while you are updating the game. It will finish, it is just longer than most to complete.
Hope this helps.