Grand Master Pabris:
To become an Elven Knight, you need the appropriate level and token. Your level must be at least 20 to qualify to change profession. You need a token to prove you have passed the Test of the Elven Knight, as well.
However, it seems that your level is not high enough, and you have not passed the Test of the Elven Knight, either. First, try to raise your level to 20 or higher. If you want to know about the Test of the Elven Knight, go to Master Sorius, in the town of Gludio. You must pass his test and bring back the Elven Knight Brooch as proof.