Why can't I turn on Norma's Fire Wall?

So I just got Norma Beatty in my party and looking at her Eres I noticed that her Fire Wall was set to off so she wont use it in battle. the problem is that I can't turn it on unlike Grave.

Fire Wall costs 12 TP and Norma has 121 TP Max so she should be able to cast it. So why can't I turn it on?

Solution 1:

According to the Wiki about her Eres:

You need 30+ "Galf" eres stones and 50+ "Eggtopus" eres stones.

Which the Wiki explains when talking about Eres...

Crystal Erens must master their eres before using it. They accomplish this by gathering "Eres Stones", the crystallized life force of monsters, which enables them to access the power within the eres scrolls. Eres Stones are gathered by defeating enemies of a certain race.

And here's the Wiki's Monster Book so you can find the monsters of the previously mentioned types (Monster Families).