Someone lied to you. You can save things to the card, just not apps. Not unless your tablet is running at least Android "Marshmallow" and the option wasn't restricted by the manufacturer. That does happen.
To save photos, music, videos, etc., to the SD card you just have to go into the settings for the app you are using (say camera) and set it to save new ones (photos) to the card. The same goes for other apps you can install to view/organize your files.
Now if you want to move all those kinds of files to the card, the fastest way is actually to connect the tablet to a computer via USB cord, move the files to a folder on the computer and then copy them back over to the SD card (via the same USB connection).
Now if you are talking apps, you would need what I stated originally, plus the following instructions... If the instructions don't work for you, then the manufacturer restricted the option on your device...
How to turn the "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.
How to turn the "External" SD card into "Internal" storage on "Nougat".
IMPORTANT NOTE: Be sure you start with a blank SD card. When the card is converted to "Internal" storage, it will be formatted (wiped) and encoded (making the cards data only readable by that device), so anything on it prior would be gone.
1. Go to "Settings", and then select "Storage & USB".
2. At the bottom of the list you should see the SD card's details, including the option to format it and make it "Internal" storage.
3. Once this is done, reboot the device and you can start running things from the card.
If you don't reboot the tablet, some things may not work correctly, so make sure you do the reboot.