Hi,
check settings you done or not ?
Following are the steps
1) "Autom. PO" field must be checked in material master
2) "Automatic Purchase Order" field must be checked in vendor master
3) Outline Agreement or Purchasing Info Record shall be maintained which will be used for pricing information
4) The Source List shall be maintained for the nominated vendor which was shown in either the Outline Agreement or Purchasing Info Record. If you are not using source list, vendor needs to be assigned with purchasing organization to the PR
5) PR should have been released
A requisition item cannot be converted into a purchase order in the following circumstances:
- If the master data is faulty or incomplete
Example: The vendor is locked or the current purchase order price is not available.
- If input data is missing (for example, unknown account assignment)
check below path is for default po document type.
Define Default Values for Document Type for transaction ME21 in IMG.
IMG->MM->PUR->Define Default Values for Document Type