Well that would be because that tablet comes with Android "KitKat" and you need at least "Marshmallow" to make the SD card internal storage. If you can somehow upgrade to "Marshmallow", then it should work, unless of course the manufacturer has restricted this option, which they do that more often than you might think. If you are able to upgrade to "Marshmallow", the instructions for converting the card are 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”...