OK Is it that the SD card isn't being accepted by the phone, or is it that you can't get it to accept apps, etc.? If it is the first one, it could be a few things from a bad card, to dirt in the port.
If it is the second one, then you will need the following...
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 been restricted by the...