I'm getting kinda worried cause i'm in chapter 6 and I can't seem to finish Aht's impurity cleansing quest :/
I don't have the recipe for the Impurity Cleanser so I'm pretty much stuck.
Is there a minimum level Aht needs to be?

Assuming you mean Quest 25 (Aht's Impurities), the requirements are:
Ch 5, Wil Lv18, Aht Lv15
The recipe should show up under the Key tab as long as Quest 25 is active.

There is no fail for the quest.

thanks that explains it. Aht was infuriatingly annoying to train so i left it/her at level 10. Again, thanks