Well, that would be because the Android version you are using does not allow for making the SD card "Internal" storage vs the standard "External". That would require "Marshmallow" which is Android 6. Anything lower would not allow for the needed change.
Now if you can somehow upgrade the phone to use the "Marshmallow" version, then you may be able to make the change. It will also depend on if the manufacturer has restricted the option. However, if you can't do the upgrade to "Marshmallow" then this will not 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 phone.
NOTE: If you don't reboot the phone, many things may not work correctly, so make sure you do.