High Priestess Levian:
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. To take the test, go to Priest Manuel, who can be found in the sanctuary of the town of Gludio. To get to the town of Gludio, follow the road to the east. You must pass his test and bring back the leaf of the Oracle.