Well first, you have to be running Android "Marshmallow" (a.k.a. 6.0). So you would have to be running that, or able to upgrade to it for your tablet to allow the change. Then, the second issue is if the manufacturer has restricted the option on that device or not. Sadly, sometimes they do.
First make sure you are already running or able to upgrade to "Marshmallow". Then try the following instructions to see if the tablet will allow for the change.
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...