Well one reason may be that your phone comes with Android "Lollipop" and you need to be using "Marshmallow" to have the SD card set as "Internal" (which needs to be done when it is put in the phone) rather than the standard "External".
Your phone 'should' be upgradable to "Marshmallow". If you wish the card to be usable in that fashion then you will need the upgrade. If you can get the phone to upgrade, and the manufacturer hasn't put any other restrictions on the phone, then you need to have the SD card set to "Internal". You will need to start with a blank card. Why? Because when it is formatted to make the change, and make it "Internal Storage", it will wipe any data that was on it, as well as encrypt the card to work with only that device.
First you need to try and upgrade to "Marshmallow" otherwise the rest is not possible. Once you have upgraded then you can use the following info to set the card to "Internal".
How to turn external SD card into internal storage on "Marshmallow".
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.
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