Please be sure you are passing the seller ID (sid) value as your 2Checkout account number not your 2Checkout username. This is most likely the problem you are having. If this is not the issue, then submit a Help Desk Ticket below and give us a link so we can do a test purchase on your cart. If possible, include the purchase link or form you are using to send the parameters to our system.
You can open a support ticket here
