When facing the error SAP Purchasing organization XX not responsible for plant YY while creating a Purchase Info Record (PIR), the root cause is most likely pretty simple : the purchasing organization hasn’t been defined for the plant. See detailed discussion on SAP.com
As described in the Performance Assistant, some customization must be missing – if the values used are correct only, of course, as it might be that wrong values are being used.
In transaction SPRO, find Enterprise Structure > Assignment > Materials Management > Assign purchasing organization to plant
Here, add an entry with the required POrg and the Plant.
Select a customizing request
And the new entry should now be there, therefore not having anymore the issue while creating a PIR