The most common cause for this error is the payment method passed in your REST API request.
"payment_method": "credit_card" VS "payment_method": "paypal"
The payment method can be "paypal" for payments with PayPal accounts and cards (where the buyer has to fill-in his card details on PayPal checkout page and not on your website).Or it can be "credit_card" for direct card payments where the buyer can provide his card details directly on your website (not on PayPal checkout page).
The "credit_card" value is reserved ONLY for PRO accounts in UK, US and Canada. In case you receive this error and is due to this then you have 2 options: