Well you have to be running at least Android "Marshmallow" and the phone has to have not been restricted by the manufacturer. Which does happen. Try the following instructions. If they work for you, great. If they won't, or any step is missing on your device, then the manufacturer has restricted this option.
How to turn the "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 encoded (making the cards data only readable by that device).
1. Go to device “Settings”, then select “Storage”.
2. Select your "SD Card", then tap the “three-dot menu“ (top-right), now...