Po delivery instructions
What is the transaction code and in which TAB we should define PO delivery instructions.
If you are talking about Shipping Instructions then configure it in SPRO > MM > Purchasing > Material Master > Define Shipping Instructions
Then assign it in Material Master "Purchasing" view, it will copy it to Purchase Info Record from Material Master and then in PO.
If you want to maintain it in PO then maintain the same in "Delivery" Tab of PO at Item Details level.
If you want to maintain the Delivery Instruction text item wise then maintain it in "Delivery Text" under "Texts" Tab of PO at Item Details level. If it is common for all the PO items then maintain it in "Terms of delivery" or "Delivery" Text in "Texts" Tab at Header level.
The above texts will get printed on the PO output if you are using SAP standard SAPScript and Print program, if you are using a customized one then take help of ABAPer to get it on the PO Printout.