In order to acquire this knowledge you need to complete a quest:https://bdocodex.com/jp/quest/3550/9/?sl=1 ; starting point of the questchain leading to this quest is https://bdocodex.com/jp/quest/3550/1/ . To atleast start this quest you'll need alchemy skilled 1 and during the questchain you'll get asked to craft multiple thingy, nothing hard or extremly expensive in fact; If somehow alchemy is annoying to you,keep in mind that Shais do start with alchemy pro 1 and are then ready to do the quest as soon as they are high level enough and done with main quest/amity with the quest giver.
Be aware that you can actually get this knowledge only ONCE PER ACCOUNT!!! So please use everything you can in order to achieve higher rank grade (elixir/pet/event bonusses...)! If somehow you get a low grade, well you are not alone sadly.In order to change this system i made a feedback on the official forum , you can got there and support the feedback by adding one upvote , and maybe one day PA will allow us to "reroll" this knowledge.

NB:this is the case for some knowledges and i extend the feedback as soon as i discover another knowledge in that case; already made a ticket for that and the GM just told me to go to feedback section. If in any case you deleted the knowledge, try to ask for a new one in a ticket as its your sole solution(which i can't guarantee).
Why getting high grade is important: Decreased damage received when the target hit you; Deal extra damage to the target (which can be interessting in order to achieve DPS rank against world bosses);Extra drop rate (even if it may or may not work against world bosses);Ecology point system;personal completion.