Klarna Checkout (KCO) – “Can not complete order” & “Invalid PNO” Error – Magento 2

I have installed and am using Klarna Checkout (KCO) on our Magento 2 default development site.
However, I encountered an issue when trying to place an order. When clicking the “Pay Order” button, a popup appears with the message:

“Sorry, we weren’t able to approve this purchase. Please choose a different payment option.”

I checked the response for the request:

https://js.playground.kustom.co/kco/checkout/orders/59d77f3e-8957-10a8-9f1f-8331b4e148b1

It returned the following error:

"errors": { 
  "generic": [ 
    "can_not_complete_order", 
    "invalid_pno" 
  ] 
}

I have checked the configuration and customer information but still couldn’t figure out the cause. Has anyone encountered a similar issue and found a solution?