Hi Lucy,
The 'Self-enrolment' Enrolment plugin gives the learner access to the Seminar/F2F via the course enrolment, whereas the 'Seminar direct enrolment' gives access direct (it an be enabled under Site admin>plugins>Enrolments>Manage enrolment plugins).
What you are seeing is the learner not being able to enrol in the course hence: 'you do not have permission to enrol in this course'.
When 'Self-enrolment is enabled' then they can, and therefore can access the Seminar/F2F activity.
Conceptually its kind of useful to think of it like this:-
When 'Self-enrolment' is enabled they can access the course, and then the activities within it.
When the 'Seminar direct enrolment' is enabled they access the Seminar directly, and are enrolled into the course as a result.
Hope this is useful,
cheers,
George.