Hi,
Hope you are using pipe line material / material type without quantity update.
If you want to maintain the stock, you may maintain the material type with quantity update for the plant. You may create new material type / change the existing material type.
If you doesnt use material master data and using account assigned PO, you have to go for master data maintenance with appropriate material type.
Please check and revert back with your issues / concerns.
Regards,
AKPT