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.
However, it seems your level is not high enough, and you have not passed the test, either. First, raise your level to 20 or higher. To take the Test of the Assassin, you must find Triskel and pass the test that he gives, bringing back the steel heart as proof. He is always near our guild, so you can find him easily.