Downloading an older version of an app
I have an older iPhone 6S. I get a message saying:Download an older version off Fitness Pal. Tap download: Nothing. How do I locate an olderversion?
iPhone 6, iOS 12
An iPhone 6s can update to iOS 14. If your iPhone is running iOS 12.5.4, then its an iPhone 6, not a 6s.
Anyway, If you have never downloaded the app before, and don't have a device that supports the current version, then there's no real way to get a compatible version.
If you have downloaded it previously on the Apple ID being used by the iPhone 6 or a have a device using the same Apple ID as the iPhone 6 that can download the current version then you can re-download it from the Purchased History on the iPhone 6 and it should offer a compatible version if one is available.