That would be because adding an SD card doesn't automatically make it "Internal" storage. It is "External" storage. There are some steps one must follow to make a card "Internal" storage. I will include them below. However, that said, if memory serves that particular device doesn't allow for making this change.
How to turn the "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...