Hey there, MonkeyGirlZoe.
It appears you're having an issue with the payment method you use for iTunes Store and App Store purchases. You received a message that you had an invalid payment method, which you tried to fix. At this point, however, the payment method has been changed to "none" and you're unable to edit it. I'm happy to help.
Payment methods that you can use with your Apple ID -- This article provides excellent information on which payment methods are supported.
If your payment method is declined, or you have a billing issue with a previous purchase in the iTunes Store -- If there was a billing issue, review the information in this article.
Change or remove your Apple ID payment information -- If the payment method you wish to use is accepted and there were no billing issues, try to update your payment method on a computer that has iTunes installed on it, or the web.
Let me know how that goes.
Take care!