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.
I am satisfied with your level, but it seems that you have not passed the Test of the Assassin. To become an Assassin, find Triskel and pass the test that he gives you and bring back the Iron Heart as proof. He is always near our guild, so you will find him easily.