I have noticed that in my minecraft pe creative mode, I can't spawn mobs. I'm not sure if it is because I am in flat lands or I need to update it.
As mentioned in the comments, the most likely cause of this issue is due to your difficulty being set to "Peaceful".
A good way to test this is try and spawn a sheep or a pig. If you can spawn passive mobs, but not hostile (like creepers or zombies), then you are playing on Peaceful.
To change this, go into your options menu, and on the first window, move the "Difficulty" slider to the right. This will then change the difficulty.
Once you have done this, try again. You should be able to spawn hostile mobs now, as they are only unavailable in Peaceful mode. They are available in all other difficulties.