Not running Android "Lollipop". You would need to be running at least "Marshmallow". Now if you can get the phone to allow you to upgrade to "Marshmallow" then it will just depend on if the manufacturer has blocked the option or not.
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 you can't upgrade/get it to work, then you can free up a lot of space by moving your photos, videos, music, etc., over to the SD card.