Site Permissions links to the SharePoint site settings. Typically, there will be an Owners group for administrators, a members group for help desk staff, and a Visitors group for end users. When the site is created, the Visitors group gets read-only access; this will need to be updated to at least edit access so that end users can submit tickets. See our documentation here for setting up for permissions with the most restrictive access:
NITRO Security Settings allow you to redirect end users to the Employee Portal if they try to access the backend.
Refer below article to set up the NITRO Security Settings.
NITRO™ Portal > Classic UI Portal > Portal Administration > Security Settings (crowcanyon.info)
