Grand Master Tobias:
To become an Assassin, the appropriate level and token are required. Your level must be at least 20 to qualify to change profession. You also need a token that proves that you passed the Test of the Assassin.
It seems that you admirably passed the Test of the Assassin, but your level is not high enough. Level 20 or higher is required to change profession. I cannot allow you to change profession if your level is not sufficient, even if you have a token. Come back when you have raised your level.