Some manufacturers set up with software which has been modified from the regular Android version and so do Service Providers. That said, it won't let you set Internal Storage because that is the default setting so you should be looking for Portable Storage.
Anyway, here's a way of taking more control over your device so go through this procedure.
Tap your Apps square and go to Settings. One of the entries is About phone; go into that and tap Build Number seven times. When that's done, you see a message saying you are now a Developer and an extra menu appears named Developer Options.