In QuickBooks Online Organisations you can hide a Category and if that's the case, it's not mandatory to state a Category in Purchase Orders. But sometimes QuickBooks Online rejects Purchase Orders with the error "Business validation: you must select an account":
The reason for this could be: you have not selected the "Expense account" for the Product you're using in QuickBooks Online, or the Product/Service has been updated during the approval process.
Please make sure you have specified the Expense Account in the Product/Service in QuickBooks Online and try again to create the Purchase Order:
Please note: Any changes in the QuickBooks Online Products and Services catalogue during the approval process in ApprovalMax may lead to such errors, meaning documents fail to be created.