Hi Hemant,
This error will happen if purchase orders are deleted manually or through system background jobs either in R/3 or in APO which resulted in this. This error will not get iterated because the deletion occured in the source(origin) area(R/3 or APO) where purchase orders are created
After clearing queues, try iterating, if not goes, check whether those POs are really required for planning. If not, delete those manually from R/3.
Regards
Ankit Singhal