If the 'space' that is free that you are referring to is say on a SD card, the phone won't consider that actual space. It can't be used by apps unless the following instructions work on that device. Otherwise you can only put photos, videos, files, etc., not apps, on the card.
First, you need to be using a phone that is running at least Android "Marshmallow" (or newer) 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...