¨This application requires iOS 14.or later¨

Before you go ahead and buy a new iPhone because of the others who have replied to this thread, I would like to tell you that there is a workaround to this issue. The workaround is to purchase it on a newer device so it will be saved to the purchased section in your Apple ID, after which you can go ahead and install it on the iPhone with the older iOS version, such as iOS 12.5.7. It will allow you to install the latest compatible version instead.

Check your iPhone model & iOS version -> Go to: Settings -> General -> About.

If you have an 'outdated' iPhone that can't be updated to iOS 14, you can't, see: If your iPhone is eligible for an update, you can check by reading this Apple article: iPhone models compatible with iOS 14.

iPhone models compatible with iOS 14.7 – Apple Support (UK)