If you receive this message : Order id contains one or more orderlines with an invalid Artikkelnummer.
1. Open MyStore to check your orders and correct the line with the invalid or missing Artikkelnummer.
Once you have corrected it , re-save the order in Sendt status to re-submit the order to PowerOffice.
To avoid the error happening again, add an Artikkelnummer; open your products and add it directly for all orders: