Mindy is right. On Android 5 "Lollipop" you cannot move most apps over to the SD card. It is not set up for that option. You have to be using Android 6 "Marshmallow" to be able to do it, and even then, some manufacturers have restricted the option.
If you can upgrade your tablet to "Marshmallow" then you can try using the instructions below. Otherwise the only option is rooting the tablet, which personally I wouldn't recommend, plus can be dangerous for the phone and voids any/all warranties.
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.