Then you must have an iPhone 6s. The 2015 model.
Apple stopped supporting the iPhone 6, from 2014, last year so it can only run iOS 12. Specifically, the last available version is 12.4.8. As that version was released 14 months ago, it obviously did not include COVID related features.
But the N.H.S. Covid-19 App needs to use the Exposure Notification functions in the operating system which were created by Apple and Google. And for iPhone those are only available in iOS 13, specifically from version 13.5 onwards which was released in May to allow governments to create tracking and tracing apps. The current version is 13.7, released this month.
Because of the way the App Store works you can still see the app and can "GET" it, because that can trigger it downloading on other compatible devices. But even if it downloads it will give you an error at the end.
As I do have an original iPhone 6, I can guarantee it will not work on my phone.