When facing the error SAPPurchasing 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
Error in Purchase Info Record creation screen
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.
Purchasing organization not responsible for plant error description in Performance Assistant
In transaction SPRO, find Enterprise Structure > Assignment > Materials Management > Assign purchasing organization to plant
Purchasing organization to plant assignment in SPRO
Here, add an entry with the required POrg and the Plant.
Entry of a new purchasing organization to plant assignment
Select a customizing request
Customizing request prompt
And the new entry should now be there, therefore not having anymore the issue while creating a PIR
