Unable to download WhatsApp
1) If the download has started and then the alert pops up: I have experienced this due to lack of network connections. Sometimes you're connected to WiFi, but there is actually no connection (for example you're connecting to a public WiFi and confirmation is needed). 

Go to Safari, type in Google.com and see if Google loads in the browser. If not, find a reliable network connection and try downloading again.

2) If the alert pops up at the AppStore after you hit the GET button, and the download cannot even be started: your iPhone might not have sufficient space. Go to Settings and check your storage. Delete the Apps, Photos and Podcasts you don't need.