Payment has failed with "credit_card_not_found" error message. Why?

The "credit_card_not_found" error message is a message, specific to ShopPay payment processor. We have contacted Shopify Support about this and they said that this happens if the customer accidentally removes the credit card from their ShopPay account and thus, the credit card can't be found in their account anymore.
To resolve this, they recommend to send the customer a link to update their payment method in Shopify admin > Customers > click on this customer > Payment methods section > More actions > Send link to update payment method. After the customer updates their payment method in there, the card should get linked back to the subscription in Shopify.

But if this still doesn't work, then the best way to resolve this will be to simply create a quick checkout link in Seal Subscriptions > Subscription rules > Quick Checkout Wizard with these items already in the order and send it to the customer. This way, they will simply start a new subscription in your shop and they can cancel the old one.