To get a Heavenly Core, Dalvin would have to find an extra Dao Rune of his own. After all, the cultivation techniques from the store were sourced or created by Shi Feng, an Immortal King. While he was talented and had the resources to create high-quality techniques, he would not invest the best for the players of Earth whom he was about to conquer.
Players — and Dalvin, for that matter — were even lucky that the cultivation techniques did not have any hidden backdoors or fatal issues. To expect to have techniques that could both lead to the Ascension Rank and possess the best dao runes necessary to create a Heavenly Core was unrealistic.
However, this was not a problem for Dalvin. He had the extra Dao Rune necessary to make his core a heavenly one, the Rule of Living Wood he had comprehended himself!
All he had to do was inscribe it on his core and he would have the necessary Dao Rune to achieve the peak!