The 800 Contract not found error indicates that the shopperReference used to complete the payment does not exist (anymore).
For example, the saved payment method could have been deleted by the shopper via their account in your system, or an incorrect shopperReference is submitted in the payments request.
In any case, it means that the current shopperReference does not have any paymentDetails saved. This can also happen if a token is created and an attempt to complete a payment, using that token, very shortly after creating it.
To troubleshoot this issue, please check the saved payment details for the shopperReference , and check which recurringDetailReference are returned in the storedPaymentMethods array.