不能還丹 You Cannot Return The Cinnabar
壽星笑道：「我因尋鹿，未帶丹藥。欲傳你修養之方，你又筋衰神敗，不能還丹。寿星笑道：「我因寻鹿，未带丹药。欲传你修养之方，你又筋衰神败，不能还丹。
逐句解 Line by line
The Longevity Star said with a smile, "I came out looking for my deer and brought no elixir with me.
丹藥, cinnabar medicine, is the pill of Daoist alchemy. He is not refusing on principle; he simply did not pack any, having come out on an errand.
I would pass on a method of cultivation to you, but your sinews are weakened and your spirit is spent, and you could not return the cinnabar."
還丹 is huán dān, returning the cinnabar — not the hái of "still" or "also." It is a technical term from inner alchemy: the practitioner circulates his own vital essence back up through the body to reconstitute an elixir inside himself. It needs a body with something left in it. 筋衰神敗, sinews decayed and spirit defeated, is a diagnosis, and it is the answer to the question the king has been asking for three years.
The refusal is medical rather than moral, and that is what makes it land. The god does not tell the king he does not deserve long life. He tells him there is not enough left of him to work with.
It also settles the question the whole episode has been built on. Inner alchemy requires your own body to be the crucible. The king wanted a shortcut that used somebody else's children instead, and the reason he wanted one is precisely the reason it could never have worked.