Hi John,
To my knowledge, the latest iOS version that you can use with the iPad Mini 3rd Gen is 12.5.5. Unfortunately, higher versions (13/14/15) are not possible.
By the way, I'm having similar issues with my "good old" iPad4. (Here the latest possible iOS version is 10.3.4...)
To use the app you want, you'll need to get a newer iPad that meets the minimum requirements (iOS 14).
Best regards,
Chris