Problem: Cannot restore a configuration backup using the GUI in OPNsense

Error: You do not have the permission to perform this action.

Resolution: Remove all access privileges leaving <priv>pages-all</priv> in /conf/config.xml for the root user

Doing it with the GUI

System => Access => Users => Edit root

On the Privileges field click Clear All and then Important! Make sure you select "All Pages" before saving the Edit User dialog

Before:

After: