Location:
In quest book, chapter 16 (Diamond from Dirt), quest 'Thermal' (Tier 3 seed row).
"... You'll need them to get Menril Seeds ..."

Exact fix:
The thermal elementals' seeds are used to craft Mithril seeds, not Menril seeds. The quest description should change Menril to Mithril.

Fixed the typo, changed it to "Mithril" from "Menril" in 2.5.0: https://www.curseforge.com/minecraft/modpacks/divine-journey-2/files/3472385