It has come here as it is related to mass deletion of PRS
You can go to SPRO do setting as shown below:
SPRO-----MAT MGMT------PURCHASING---ENVIRONMENT------ATTRIBUTES OF SYSTEM MESSAGES------------SELECT MESSAGE 06050.SET IT TO ERRO INSTEAD OF WARNING
GO TO SU01
SELECT USERS WHO CREATE PRS.GO TO PARAMETERS
ENTER PARAMETER MSV ,MAINTAIN PARAMETER VALUE 06050 & SAVE
Select one by one or all.Then select delete.
Check in ME53N. PR deletion indicator is set.
User can not convert PR to P.O.
If attempts to create P.O. error message pops up