MC was a disciple of a powerful swordwoman. He has learned her master's sword skills (enough that he's allowed to teach) and inherited her OP sword, but he cannot touch that OP sword and is only allowed to teach the sword. Why?
We can see the reason by looking at his performance in this chapter.
This was supposedly a serious fight, and MC needed to finish the fight quickly and move on. And yet MC acted like he's performing in front of camera, kept posing while dodging and watching the enemy, dragging the fight for no reason. We didn't even see the form of his weapon yet because the oh-I'm-so-cool MC finally decided to start morphing his arm at the very last page.
No wonder the OP sword considered MC unworthy.