The error that shows on a requisition as "The line item contains one or more invalid flields" is most likely due to the Description of the line item. If the Description on a line item is more than 240 characters, it will trigger an error to the preparer, but many times the preparer can't figure out why.
To resolve: Click on the Approval Flow tab. Instead of the normal approval flow, you will see an error that says "The approval flow cannot be displayed because the Ariba system found the following error or missing fields." Below that, the line item with the description causing the error will be listed, along with a better description of the error. Here is an example: "Line Item 1, Description: Full Description can be no longer than 240 characters." The preparer must edit the description down to 240 characters or less in order to submit the request.