Why can’t I download WhatsApp on my iPhone 11

See -> 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 the following Apple articles:

Payment methods you can use with your Apple ID

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

Add money to your Apple ID balance

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

Also note that WhatsApp is unavailable in certain countries due to governmental regulations. Can you see it in the App Store?