QBO Error: A business validation error has occurred while processing your request: Business Validation Error: Transactions can have only one foreign currency at a time.

This error comes up if the customer/supplier in QuickBooks does not match the currency of the invoice or if it does not support multi-currency. 

Please ensure the assigned Currency on the invoice should also be the same in your QuickBooks account. Please note that a QBO customer/supplier can only have one currency.