Hello,
try this:
If text type "F18
" does not yet exist, you must create it in Customizing for Materials Management under 
Purchasing
Purchase Order
Texts for Purchase Orders
Define Text Types for Header Texts
. You have to enter the Sequential Number "18".
If your company already uses text type "F18
" for a different purpose, you can use the BAdI Extend Runtime of Gateway Service for Approve Purchase Orders App
(GBAPP_APV_PO_RDP
), method SET_ATTRIBUTES
, to use a different text type to store the notes. This BAdI is available in Customizing for Materials Management under Purchasing
Purchase Order
Approve Purchase Orders App
BAdI: Change Data of Approve Purchase Orders App
.