The app using some of the new APIs that are only available on iOS15 and WatchOS 8. Those were released on Monday, and so was Runervals!
Most of my target audience will not have the latest operating systems installed yet, so won't be able to find the app, but I've done a mini launch on twitter, where most of my network is other iOS devs.
I plan to follow this with a product hunt launch once a larger percentage of the population has upgraded to iOS 15.