There are two kinds of authorised exam centres:
Open centres are authorised to run examinations for all candidates, including those who are not taking a preparation course at the institution that runs the centre. Many open centres also run preparation courses for Cambridge English exams.
Internal Centres are authorised to run examinations only for students enrolled at their institution. They are not able to accept entries from candidates who are not their students.
Some centres are 'open' for some exams and 'internal' for others. If they are unable to accept your entry, for this or any other reason, they will inform you.