Hi,
This error may have chance to come if you didn't maintain your output device in particular condition record.
we can maintain printer in two places.
In output conditon record for purchase order in transaction MN04, check have you maintain the output device for your respective condition record or not.
or
Transaction OMGF, maintain printer against your pruchasing group.
If it's not maintained plese maintain and let us know wheatehr this information helpful to solve you issue or not.
Regards
Karthik