This error is not a Spry Fox issue; it is an issue with the device, the operating system, or the SD card. You can see this if you do an internet search for "Couldn't install on USB storage or SD card"; there will be many results that don't involve Spry Fox games.
The internet discusses several approaches to fixing the problem, one of which is the following:
- Go to Settings >> Storage >> Unmount SD Card
- Install the app
- Remount your SD card.
Unfortunately it appears that this solution doesn't work for everybody, so you may have to look at some of the other search results as well to find something that works. If all else fails, please contact either your manufacturer's or Google's customer support and hopefully they'll be able to help you resolve the issue.