There is no operation available to retract 'internal' publications. Add an operation to retract 'internal' publications and add it to the corresponding retract workflow, which is started by the new admin UI.
PR 627 (https://bitbucket.org/opencast-community/matterhorn/pull-requests/627/mh-11014-added-several-bugfixes-and) has introduced to new WOH: publish-configure and retract-configure.
WOH publish-internal has been replaced by WOH publish-configure and WOH retract-configure adds the missing ability to retract internal publications.