High Priest Sylvain:
You need the appropriate level and token to become a Human Wizard. To change profession, your level must be at least 20, and to become a Wizard, you must have the token that proves that you passed the Test of the Human Wizard.
However, it seems your level is lower than 20, and you have not yet passed the Test of the Human Wizard. The first thing you need to do is train harder to reach level 20. If you want to know about the test, look for Parina, who studies elemental magic at the temple in the village of Gludin. You must also bring the bead of four seasons, which she will award you when you collect the tokens for all four elements.