Grand Master Ramos:
To become a Human 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 Human Knight, as well.
However, it seems that your level is not high enough, and you have not passed the Test of the Knight, either. First, try to raise your level to 20 or higher. If you want to know more about the Test of the Human Knight, visit Sir Klaus Vasper, who is high among the order of Knights. He will be somewhere around this guild. You can easily find him outside. You can change profession to Knight if you pass his test and bring back the Sword of Ritual.