Can I first clarify if you are a merchant who is trying to manually mark an order as paid? Or are you a customer trying to pay for an order?
If you are a merchant, can you let me know how you have set up your manual payments in the admin section? Also, do you know if the payment method is cash on delivery, bank order, etc?
If you are a customer and are having difficulty completing your order, it is best to reach out to the store owner directly so they can investigate the error you are seeing.
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