First, your phone must be running Android "Marshmallow". Second, your device must not have been restricted by the manufacturer. If it was, then the options just won't be there. There is nothing anyone can do about this. It would be something the manufacturer did. Which happens way more often than you would think. The instructions to make the change are below. Again, if you are missing any of the items listed, then the manufacturer restricted this option on your device and you would have to talk to them.
How to turn external SD card into internal storage on "Marshmallow".
NOTE: Please be sure you start with a blank SD card. It needs to be blank because when the card is converted to "Internal" storage, it will be formatted (wiped) and...