How can I still get in-app purchases when my bank details have been deleted from ipad

If you've deleted your payment details from your account (payment details are tied to an account, not a device) and want to do in-app purchases, you will either have to add your payment details back onto your account or, and if they are available in your country, redeem iTunes gift cards onto your account so that you have a way of paying for them.

If you want to add your payment details back : Change or remove your Apple ID payment information in the iTunes Store - Apple Support