SAP Message VV156 - Priority for 'document number' must be higher than for 'document item'

Diagnosis While evaluating the sorting criteria for rescheduling, you haveevaluated 'document item' higher than 'document number'. System Response Sorting is not allowed. Procedure Allocate a higher priority for 'document number' as for 'documentitem', where '1' is the highest priority and '5' is the lowest.