ERS Checkbox in Purchase Order

Hi all,

I've activated ERS for my vendor in vendor master and the ERS indicator is now shown in purchase orders for this PO and MRRL works fine. However, the ERS checkbox is disabled (not user editable) in ME21/22/23N.

There is a business request to allow flexibility for users to check/uncheck ERS at PO at their discretion. Where can I enable that check box? Can't seem to find any configuration/field status for it. Refer to screenshot for the exact check box I'm talking about.

Thanks!