If I'm right, that model runs Android's Marshmallow flavour so you would need to declare the storage either Internal or Portable storage in Settings>More>Storage.
Before you can Root you need to go into Settings>More>About Device and tap Build Number seven times. You are then a Developer and your first task is to enable USB Debugging.
That done, you can download KingoRoot on to your PC and connect the phone with a USB/Micro USB cable. Run KingoRoot and you're done but it doesn't need to be rooted to carry out that Storage allocation function.
EDIT: It should be made clear that manufacturers and retailers of mobile 'phones frequently mess with basic Android systems to suit their own reasons so if they don't want you to do...