Hi Lethiwe,
Note that it will only show Quote Number when you make the purchase order from Quote.
Convert the Quote into Purchase Order by clicking Make Order

In Purchase Order

The enum field "Purchase Order Document Type", and "No." fields are the key fields in Purchase Header Table..
When the "Purchase Order Document Type"::Order, the "No." is PO number, and when "Purchase Order Document Type"::Quote the "No." is Quote.
If you're creating the Order from Quote, you can easily get the Quote number in the Purchase Order header table.. just add the condition in the PO header and get the Quote number manually by setting "Purchase Order Document Type"::Quote
Regards People Eater