The sales order field links Sales Orders (SO) created in manager to Sales Invoices. They’re not the field to enter manual customer Purchase Orders (PO) references.
The manual field existed before the linking was introduced and they were kept for a while to ease the transition. Anyway, the old Order # field functions exactly like a custom field, so there’s that.
Possible solutions to your issue are:
Use customer PO as the reference for your SO and just copy the SO to an invoice.
If you don’t want to use the built-in SO feature, you can use the description field to manually enter customer PO ref.
If you’re using the description for something else, you can always create a custom field to hold manual customer PO ref.
@sandralord, this was the subject of a lengthy lead article in the July newsletter. If you do not subscribe, this is an excellent example of the newsletter’s value. You can subscribe at the bottom of this page.