Print the pick instruction
The standard report shows a QR code of the GTIN on each item line.
From the sales order, print the Pick Instruction report. In a German interface the report is Kommissionieranweisung.
Each item line on the report has a QR code. The code contains the item GTIN, not the item number. If the item has no GTIN, the code is empty.
Use that printout to scan in the warehouse, then copy the quantity onto the order as described in Copy the quantities.
