sory the the noob question but how do you upgrade from serpent corporal to serpent sergent 3rd class please?

when i try to see what is needed for promotion like previous ranks... it just says im not eligble for promotion or somethin


i googled it but this is all i found n cant see quest anywheres in game ;/

http://ffxiv.gamerescape.com/wiki/Gr...the_Twin_Adder