Q: Not able to add debit card to my itunes acc. Kindly help

Depending on where you are located, the quote below from Payment methods you can use in the iTunes Store, App Store, and iBooks Store - Apple Support may be a possible explanation:

In countries where the iTunes Store only sells apps, the accepted payment methods are Visa, MasterCard, and American Express. Other payment types such as gift cards, store credit, monthly allowances, ClickandBuy, and PayPal aren't accepted.