Well your phone should be running "Marshmallow" which, as an OS, does allow for the SD card to be converted to "Internal" rather than "External" storage. However, that said, some manufacturers have restricted that capability on their devices. To try it out you would need to start with a 'blank' SD card. Why? Because when the card is converted over it is formatted, which will wipe all data on it, and then encoded to work with only that device. So, once you have a blank SD card to try, use the following instructions, and if your device does allow for the change, you will then be able to use the card as "Internal" storage as well.
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.
If this doesn't work on your device, then you can only use the card as "External" storage, which will not allow for apps to run from the card. You can free up space on the phone though by moving photos, videos, music, etc. over to the SD card, freeing up space on the phone.