Grand Master Xenos:
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 also need a token to prove you have passed the Test of the Palus Knight, as well.
I am satisfied with your level, but it seems that you have not passed the Test of the Palus Knight. Go to Master Virgil in the town of Gludio to take the test. You can change profession to a Palus Knight only after you take his test and bring back the gaze of the abyss as proof. You can get to the town of Gludio by following the road towards the east. Of course, using a Gatekeeper is faster.