I assume you've tried this in more than one outlet? I've know it to fail from time to time for no apparent reason, even when the shop starts the whole process from scratch. The fact that the terminal is beeping suggests that a connection is being made.
TBH, I thought the app would tell you if the device wasn't compatible, but there have been several updates since I started using it, so maybe it doesn't now?
Could you try adding a different card and see if it works then?
