Grand Master Xenos:
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, bringing back the iron heart as proof. He is always near the Dark Elven Guild in the town of Gludio, so you will easily find him nearby once you get there. You can get to the town of Gludio by following the road towards the east.