how to restrict user not to submit purchase order without product fiscal classification

Hi,

we have made setup for tax based on the product fiscal classification. When users create a PO from the PR, the PO lands in the lines tab, and product fiscal classification is in the schedule tab (which is already mandatory), but if the user doesn't click on the schedule tab, it means it is allowing the user to save and submit the po. Need to restrict the user not to submit the purchase order without product fiscal classification.

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Regards,

Sazeeth Singh R