Lin Jin wasn't obligated to engage in an apprenticeship inside the Friar Association.
The Museum of Deadly Beasts provided him with many methods to cultivate his blood contract and they were all superior techniques that stretched back to ancient times. So why should he get himself a mentor?
If he did, it would only serve as a restricting spell, limiting his freedom of movement.
It wasn't worth it.
It wasn't worth it at all.
Not to mention the fact that he'd run the risk of his secret being exposed.
Hence, Lin Jin's current strategy did not include getting into an apprenticeship. From now on, he would keep a low profile until the day of the disciple's tournament when he secures the dragon blood pill.
After that, he could even drop out of the Friar Association.
However, that mentor, Luo Beihe did remind Lin Jin not to underestimate his opponent.