You must have a prepaid account with sufficient credit to carry out the operation (keep in mind the VAT, if applicable). The prepaid account being credited must be the same user who activated the auto-renewal (person requesting the operation).
For Example: Your domain has owner contact AB9234-GANDI and billing contact CD9678-GANDI. If AB9234-GANDI activates the auto-renewal for the domain, that will be the contact that must have sufficient prepaid credit. We will not debit the prepaid account for CD9678-GANDI, even if there is enough money in it.
Since Gandi does not store your credit card information, automatic renewals are made via your Gandi prepaid account.
In order to assure that your automatic renewals will be taken into account, it is necessary to ensure that your prepaid account has enough funds to pay for your renewal operations if you are not using automatic prepaid account crediting.
See here for instructions as to how you can add credit to your prepaid account.