A subadmin is logging in and able to get up to the menus to do their job but on first login they are getting a permission error.
If a subadmin is not correctly configured to see the correct content, they will get a permissions or security error when they log in. Depending on which kind of subadmin the admin is, you will need to do one of the following to fix it:
If the subadmin is a center admin... If you have multi-center, your center admins should see a special center page when logging in. It is the same page a site admin sees when he switches to that center. If your center admins are not seeing that, then you need to give them "View and User Centers" permission in the "Center Management" section.
If the subadmin is not a center admin... If you don't have multi-center or you have configured an admin to have access to multiple centers, they should see the PageBuilder page called "AdminHomePage" when logging into your site. This is the same page a site admin sees everything he logs in. If your subadmins are not seeing that, then you need to give them "View Content" in the "Content Viewing" section. You can only do an override for "Administrators only" if you'd prefer as that is the security category that the "AdminHomePage"page is in.