Actually "Internal" storage is the storage inside the phone, which should allow for apps just fine. "External" storage, which is what an SD card is, in most cases won't allow for apps to be installed to and run from them. There is the possibility that you can get the SD card converted to act like "Internal" storage, maybe.
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...