Ok, assuming it's saying name or password are wrong, try resetting your game password. When logging in to game, if you don't have the login set to remember, you'll get a Forgot Password reset button that should email a new one to your entered email. If you do have it on remember, clear the app data cache, and I think you'll get the reset password option when you login next (or in the worst case, delete the app and reload it from the store, in android terms), and enter email and reset password.
Then try that new password in the name change tool.
Manfred