Why can't I find the offering I want to set up in page header menu?

You can't set up an offering that you haven't enabled yet. First, follow the steps for configuring offerings and enable the offering you want to set up.

If you don't have permission to configure offerings, then contact someone in your organization who configures the offerings and ask them to enable the offering. Once it's configured and enabled, return to the Setup page and you can find the offering added to the menu.