Well, if you are running on an OS other than "Marshmallow" you would not be able to download directly to the SD card. You need to have at least "Marshmallow" to allow for direct download of apps to the SD card.
You can download to the device and then move things over to the card that are of course movable. That will work for photos, images, videos, music, etc., but not most apps.
To free up space on the phone, you might want to move your photos, images, videos, music, etc., to the SD card. It will give you a lot more room for adding and running apps.
If you can find a way to upgrade the OS to "Marshmallow" then you may be able to change this. However, that is a big 'maybe', as some manufacturers have put a limitation on their...