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 you have passed the Test of the Elven Oracle.
You have achieved a sufficient level, but to become an Oracle, you must pass the test and bring back the leaf of the Oracle as proof. To take the Test of the Oracle, go to Priest Manuel, who can be found in the sanctuary of the town of Gludio. To get to the town of Gludio, take the road to the east. You must pass his test and bring back the leaf of the Oracle.