If you are wanting to move apps over to the card, then that works differently. You should be running the right version of Android, "Marshmallow", so I am including the instructions below on how to set it up. However, please note, that it is completely possible that the manufacturer has restricted this option on the phone (sometimes they do) and then it won't work. Fingers crossed.
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 phone.
NOTE: If you don't reboot the phone, many things may not work correctly, so make sure you do.
Now, if it is just photos, music, etc., that you want to move to the card, you can then connect the phone to a computer via USB cord and move all the files (photos, music, videos, etc.) over to a folder on the computer. Organize them how you want and then via the USB connection send them back, but to the card this time.