Can you just uncheck the “Shipped” checkbox, open the packslip, and delete it? since there do not appear to be any actual transactions, there shouldn’t be anything to prevent that.
Take a look in existing AR Invoice Groups and see if it is there… looks like someone needed to test something. The shipment screen says “Shipped”… if the invoice had been posted it would say “Invoiced”. To me that indicates the invoice has been created but not posted. If you do find it in an open AR Invoice Group, delete it and then see if you can unship the pack.
There are no open AR invoice groups. I tried creating a new one and pulling this shipment in but the shipment does not appear in any of the filter options (not shipped, not invoices, invoiced, voided, pending order completion).
Okay… now it’s circular. Your original error is because there are uninvoiced shipments… and now the error says that an invoice exists. Channeling my inner Rocky, I’m thinking yo, Adrian, this is highly irregular. To me this looks like a job for support.