Well not all phones are capable of running apps from the SD card, many can only handle moving photos, videos, music, etc., over to the card. Below are the instructions on how to do it (and what is required to do so).
First, you need to be using a phone that is running at least Android "Marshmallow" or be able to upgrade to it. Second, you will need the instructions I am including below. And third, you need a phone that has not been restricted by the manufacturer from making this change. Which does happen.
NOTE: If any of the steps are missing on your phone, or they don't work for you, then you have a phone that either has an older Android version or has been restricted by the manufacturer.
How to turn the "External" SD card into...