You can't just download this from the App Store. To get this working, you'll need:
This is arguably the safest source for finding legitimate legacy IPA files. Many users have uploaded their iTunes-purchased IPA collections over the years. Search for "WhatsApp iOS IPA" or browse the collections dedicated to old iOS apps. whatsapp ipa ios 6 best
The decision to drop support for iOS 6 is not arbitrary. There are three fundamental reasons why this happened: You can't just download this from the App Store