High Priest Sylvain:
You need the appropriate level and token to become an Elven Wizard. To change profession, your level should be at least 20, and to become an Elven Wizard, you must have the token that proves you have passed the Test of the Elven Wizard.
You have achieved a sufficient level, but it seems you have not yet passed the Test of the Elven Wizard. To become an Elven Wizard, you must visit the Elven village and speak to an Elf named Rosella. You can change profession to Elven Wizard only if you pass her test and bring back the token called the eternity diamond. Rosella is right there in the village, so it won't be hard to find her.