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 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 of the Palus Knight, either. First, raise your level to 20 or higher. Also, seek Master Virgil of our guild in the town of Gludio to take the test. You can change profession to 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.