Well, as that phone should be running Android "Marshmallow", which is what is needed to make the necessary changes, then the only other obstacle would be if the manufacturer has restricted this option on that device. It happens. To see if you can make the change, use the instructions below.
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 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 select “Settings” from in...