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.
However, it seems your level is not high enough, and you have not passed the Test of the Assassin, either. First, raise your level to 20 or higher. To take the test, you must find Triskel and pass the test that he gives, bringing back the Iron Heart as proof. He is always near the Dark Elf Guild in the town of Gludio, so you will easily find him once you get there. You can get to the town of Gludio by following the road towards the east.