Have a button/box that you can click/tick when setting up clients or amending them so that the account is noted that PO NUMBERS MUST BE SHOWN ON INVOICE - Therefore when you come to invoice you HAVE to enter a PO Number or it will not let you save or will showa pop up prompt so you can stop and go and chase the PO
You could use the supplier alerts functionality to advise the operator to enter a PO Number.
You can find out more in our helpfiles - https://desktophelp.sage.co.uk/sage200/professional/Content/SL/Customer%20alerts.htm?Highlight=customer%20alerts