Why can't I edit club orders as an Admin when the club package is configured to Set Items?

If you use Set Items, even the Admins cannot override the Package content.

If you really want to make an exception for a Club Member, you can process this shipment in the POS and mark the Order as the Club Shipment. This video shows you the process. (Note that this video is specific to brand-new Club signups, but the same process applies to those who are already Members).