How to Resolve: XML Generation Error. The request that was issued could not be understood by the QuickBooks request processor. Please check the QBXML Version to make sure your request is supported

This is a generic error message that QuickBooks provides and could indicate a variety of problems. 

First, check to see that there is a price set assigned to each order on the invoices being sent to QuickBooks. QuickBooks requires this information to properly assign service items. If a price set (or service level) is missing from any order, the above error may appear. 

You may also try importing one invoice at a time until you identify the order causing the issue. Navigate to the tracking view and open the order(s) with the issue and verify that the orders contain all information required by QuickBooks. The fields you need to verify will depend on the fields you have selected in the Post to QuickBooks Desktop dialog, as shown here: