Says so in-game, and gives me this tooltip:

I doubt you'd need the game log, but here is it just in case:
latest.log

Hi! Please try to reproduce this issue without any other mods installed, and possibly try to figure out which mod or configuration is causing it.

Hi!
I have found a very similar issue on 1.16.5.
Spent some time rebuilding my modpack (adding mods progressively) and reconfiguring it. I was unable to determine a specific mod or configuration that is responsible.
Here is the error I get in my client: https://pastebin.com/9cZE9JKF
Thanks for this awesome mod!

Did you change any of the mod's recipes without keeping the same recipe name or updating the book accordingly? This can be either crafting recipes, or any of the mod's custom recipe types.

Edit: Also, does this issue also occur if you use the default language (US English)?

Thanks for you quick reply @Ellpeck, and sorry for heisting your issue @vizthex123.
I did not change any of the mods recipes.
I had tried changing the language to the default en_us, but had never restarted the game, doing so does indeed resolve the issue.
This does pose the problem of localization nevertheless, as I was trying to translate parts of the mod (namely, the book).

it's been fixed for me, think a recent update did it but idk

If this issue still happens, and is only related to the language that is selected, please report it to the Patchouli issue tracker. Thanks!