Your customer has purchased subscription in the past. Their order fails because the card that has been saved for billing has expired.
The plugin and order management workflow does not allow for a customer to add a new card without a checkout. A checkout is the only way the order management gets updated fully about payment and billing.
Two options available to allow the customer to add a new card and continue their subscription with your business :
OPTION 1 - Customer initiated
- Customer can purchase a new subscription and checkout with the new card details.
- Customer or admin can then cancel the old order
- Customer can also delete the old card from "My Account"
OPTION 2 - Admin initiated (less customer involvement)
- Once a customer's card expires, the admin can search in the wordpress subscriptions for the customer's subscription order. (Woocommerce --> subscriptions)
- Click on the subscription order ID to edit the customer's subscription.
- Go to the Subscriptions Actions panel at the right corner of the page
- From the drop down, select "create pending renewal order"
- This will put the subscription on hold and will offer a "pay" option next the the subscription in "My Account" for the customer to checkout with the new card.
Additional info here :