You cannot move apps onto the SD card in that version of Android. You would need to be running "Marshmallow" a.k.a. 6.0. If you can upgrade to it, then the following instructions should help.
NOTE: If the manufacturer has restricted the option on that device (it happens) then even the upgrade to "Marshmallow" and the info below won't make it work.
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 there.
3. Now select “Format as internal”, and then “Erase & Format”.
4. Your SD Card will now be formatted as internal storage.
5. Reboot your tablet.
NOTE: If you don't reboot the tablet, many things may not work correctly, so make sure you do.