Location:
In the quest "Tastes like Marzipan if you had Diamond teeth", Chapter 18

Exact fix:
The quest description says that 5 Terrasteel ingots are used for the Alfheim Portal, but 6 are needed in total. Moreover, there is an extra space in the second paragraph: "To continue on, you'll need"

Now the quest specifies the correct amount in 2.7.0: https://www.curseforge.com/minecraft/modpacks/divine-journey-2/files/3486165 (It still requires 4, but the reward now gives 2.)