High Prefect Castor:
Basically, to be qualified for a Monk, your level must be at least 20 and you must pass the Test of the Monk.
However, it seems that your level is not high enough, and you have not yet passed the Test of the Monk. First, work to raise your level to 20 or higher. To take the Test of the Monk, seek the Orc named Gantaki Zu Urutu of the Orc village. He will tell you everything you need to know. After you pass the test, come to me and I will change your profession. Use the Gatekeeper to go to the Orc village.