Thanks for reaching out to TextNow Wireless support. As it turns out app developers are the ones with the ability to enable "move to SD card".
Most apps, at the choice of the developers, can only be stored on the internal storage. If you go to Setting > Apps you can go through each of your apps to see which ones you can move to the SD card.
There are also some apps that can assist with this. You could try Link2SD for free which may be able to assist with moving apps to the SD card.
Let us know if you have any questions or concerns.