The ability to add tabs is permission-based. In the default data set, the permission is assigned to Authenticated Users in the file file uportal-war/src/main/data/default_entities/permission_set/Authenticated_Users__ADD_TAB__UP_SYSTEM.permission-set.xml. It's recommended to change the group from Authenticated Users to 'Portal System' to allow admins and layout owners to add tabs. However you can adjust the group(s) as appropriate. Add multiple <group> entries for multiple groups.
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