The Grand Master is right; going by their physical constitution, humans would need to at least wait until breaking through to the Eighth Realm or Ninth Realm for their physical qualities to catch up.
In fact, it's possible to cultivate even if one's physical constitution can't quite keep up.
After all, this is a Cultivation Technique Card.
However, the issue lies in the fact that if the physical constitution can't keep up, it will be very hard to make any initial progress.
It's purely a waste of time.
Instead of doing that, it might be better to first use the Da Pin Heavenly Immortal Technique as a transition.
Then, when the physical constitution meets the requirements, switching to the Eight-Nine Profound Technique would be somewhat easier.
Although both cultivation techniques are more or less the same, theoretically speaking, the Eight-Nine Profound Technique holds a greater appeal for Jiang Yuan.
