Grand Master Tobias:
To become a Palus Knight, you need the appropriate level and token. Your level must be at least 20 to qualify to change profession. You need a token to prove you have passed the Test of the Palus Knight, as well.
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. Also, seek Master Virgil of our guild to take the Test of the Palus Knight. You can change profession to Palus Knight only after you take his test and bring back the gaze of the abyss as proof.