To be a Treasure Hunter, your level must be over 40 and you need to collect the Mark of the Seeker, the Mark of Trust, and the Mark of the Searcher. Your level is already over 40 but you still do not have the marks.
To change to a Treasure Hunter, you have to bring all 3 marks I told you about when I was talking about occupations. Now, take the tests and bring me the marks.