SAP Message C2251 - Sales order/item &1/&2 does not exist (see long text)

Diagnosis
You want to convert a planned order or carry out a partial conversion.However, planned order &V3& cannot be converted.

System Response
In planned order &V3&, you specified sales order/item (&V1&/&V2&) as anaccount assignment object. Item &V2& in the sales order &V1& has beendeleted in the meantime, and can therefore no longer be copied in theorder as a settlement receiver.

Procedure
Change the account assignment data accordingly in planned order &V3& onthe 'Assignment' tab page.