High Priest Sylvain:
You need the appropriate level and token to become an Elven Oracle. To change profession, your level must be at least 20, and to become an Oracle, you must have the token that proves that you passed the Test of the Elven Oracle.
However, it seems your level is lower than 20, and you have not yet passed the Test of the Elven Oracle. The first thing you need to do is train harder to reach level 20. Then you must pass the test and bring back the leaf of the Oracle as proof. Priest Manuel of our temple, who has taught us Oracles for a long time, is in charge of the test. If you want to know more about the test, speak to Priest Manuel of the town of Gludio.