Unable to download WhatsApp to my iPhone.

If you can't make purchases or update apps, including free apps, you need to update your payment method or add a new one. 

See also -> Payment methods you can use with your Apple ID

Payment methods you can use with your Apple ID – Apple Support (UK)

You can also use your Apple ID balance as payment method: Add money to your Apple ID balance

Add money to your Apple ID balance – Apple Support (UK)