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If you try to place a test card order against a customer who already has a live card stored as a payment method, then the test card order will be rejected with this error message.
If you want to use the same customer info to test then you should be able to go into the customer profile and remove the live card payment method. You also have the option to either delete your customer to remove everything attached, or just use the info of a different test customer moving forward.
Q: Error: Upsale item expects offer with valid Gateway or Gateway Profile.
Trying to process an upsell without a main offer and the upsell product is set to 'Use Offer Gateway' in the campaign.