Can’t download amazon prime video app

Below your question it says you are running iOS 11. If this is correct, you need to update your device. Prime Video is only compatible with iOS 12.1 and up.

https://support.apple.com/en-us/HT204204

Perhaps because it requires iOS 12.1 or later?

See this for compatibility & requirements:--> ‎Amazon Prime Video on the App Store

Regards

Giulio