Grand Master Rains:
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.
I am satisfied with your level, but it seems that you have not passed the Test of the Human Knight. You can only be accepted as a true Human Knight when you pass the test. If you want to know more about the test, visit Sir Klaus Vasper in the village of Gludin. He is near the fighters guild of Gludin. You can change profession to Human Knight if you pass his test and bring back the sword of ritual.