Enabling PunchOut
PunchOut must be enabled for the site and for each Customer that will use it. Enabling only one level does not establish a working PunchOut relationship.
Enable the site capability
Open Site Manager > Catalog Configuration > Shopping Cart and enable PunchOut.

If generated PunchOut XML should also be retained on the server, enable Save a copy of Punch Out XML to server. Configure a valid destination path before relying on that copy, and use the optional file prefix when saved PunchOut files need an identifying prefix.
Saving a server copy is a logging and support choice; it does not replace delivery of the cart to the buyer’s procurement application.
Enable the Customer
Open Customers > Customer Maintenance, edit the intended Customer, and use the Settings tab to enable PunchOut. The Customer record is also where the relationship-specific mode, action URL, transformation file, and allowed request origins are configured.
PunchOut remains unavailable for a Customer until the Customer-level option is enabled, even when the site setting is active.
After both levels are enabled, continue with PunchOut Configuration before testing a buyer session.