I want to remove my credit card details as my credit card is invalid

Try logging into your account (not when trying to download something) by tapping on your id in Settings > iTunes & App Store on your phone and tapping 'view apple id' on the popup, and see if you get the 'none' option on the payment details screen : Change or remove your Apple ID payment information in the iTunes Store - Apple Support.

You are just logging into your account, not doing it when trying to download something ?

Do any of the scenarios on this page apply : Why can’t I select None when I edit my Apple ID payment information? - Apple Support ?

(I'm just a fellow user, I'm not going to post my email address on here.)