That's because at that stage, a Professional's body condenses an Invisible Energy Core, which acts as a central hub for all their power. From then on, with targeted training, they have the opportunity to inscribe a Professional Specialty into their Energy Core with each new Great Level they achieve.
In other words, an Apprentice-level Professional like Fang Zhou had not yet condensed an Invisible Energy Core. Even with the right training techniques, it was nearly impossible to master a Professional Specialty.
However, since Principal Su Tiankuo had promised Fang Zhou in front of everyone that he could learn an Undead Mage Professional Specialty, he must have the technology and means to fulfill that promise.
