how to delete Pro Forma Sales order

Hi SAP Gurus and Expert,

Would like to check with you how to delete the Pro Forma Sales Order. Whereby the Pro Forma Invoice is created against the Sales Order.

I have already cancelled the Pro Forma Invoice, but who to delete the Pro Forma Sales Order.

This is what I have done at the document flow where the Pro Forma billing is being cancelled, I cannot delete the Sales Order and hitting error "This document cannot be deleted"

Kindly advise.

Thank you

Regards

Leonard Tan