Can’t download whatsapp

Do you have a valid payment method on file? You won't be charged to download a free app, but may still need to have the payment method just in case you decide to use an in-app purchase. See step 2 of this support article:

If you can’t download or update apps on your iPhone or iPad - Apple Support

If you have a previous purchase that has not been paid for yet, then that will need to be paid before you are allowed to download anymore apps.