Grand Master Schule:
In order to change your profession, you must reach at least level 20. That holds true for becoming a Elven Scout as well: you must be level 20 or higher, and you must pass the Test of the Elven Scout.
Although your level meets the requirements, you have yet to pass the test. Seek out Reisa in the Town of Gludio to take the test. If you pass, return here with Reisa's Recommendation as proof of your fitness to become an Elven Scout.