I can't replace a world to play in the realm because it gives me an error. I need help

I bought minecraft but i can only play the demo and i downloaded the launcher twice

When i try to spawn a endermite it despawning instantly (in creative with the egg generator and on survival mode with the ender perls) but when i try to do the same thing in other world it does'nt happendmthe endermite did not disappear PLEASE HELP