Portal Settings UI section is missing after upgrade to Madrid
Madrid and forward
The default view was modified from Out of Box version, therefore the new section is not shown in the form
Go to https://instance_name.service-now.com/sys_ui_form.do?sys_id=dffdb5c537a1010054b6a3549dbe5d25
Look at the related list called "Versions", configure the related list and add it if not present, then revert to the latest version
You can import the attached XML files to add the form section to your instance.