Can I generate a vendor purchase order from a sales order?

It is not possible to generate a purchasing document from a sales document.

You can duplicate that document as a vendor order, to do this go into the sales or work order and click on More > Duplicate and you can duplicate the that sales order into vendor order.

In the vendor order you can indicate in the Title field, the reference of the sales order, so that you can easily locate the vendor order.

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