I have been trying without success to re-download WhatsApp and every time it says "unable to download". I am not sure if it's a space issue, I would think it would tell me I do not have enough available storage. I am using an iPhone 5c with iOS 10.2.1.
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.
User needed to delete some unused data from the phone to allow the App Store to download the new app updates.