To be a Plainswalker, your level must be over 40 and you need to collect the Mark of the Seeker, the Mark of Life, and the Mark of the Searcher. But what is this, you level is not high enough and you haven't even collected all of the marks.
I told you when I was explaining about occupations didn't I? Come back after collecting the marks and after increasing your level to 40. Then I shall change your occupation in to a Plainswalker.