That’s very old device . Deezer for iPhone can works only on iOS 15 and later . iOS 6 is way old. Your only solution is to upgrade to a new phone with at least version 15 

Noam is right, see the App Store page for the Deezer iOS app at the bottom:

Deezer: Music Player, Podcast on the App Store (apple.com)

I have an iPhone 6s Plus from 2015 with iOS 15.8.2 which is the oldest device that can still install at least an iOS 15 version, as far as I know. Not sure for how long this support will be available though.