I want to download WhatsApp but it keep asking to add debit cards

See step 2 here. You will not be charged to download any free apps. If you owe money from a previous purchase, then that would need to be paid first if step 3 applies to you:

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

olalere51 wrote:

iPhone 6s, I want to download app but it won’t let me

Well what happens when you enter a valid card payment?