Ambrosia has always been hard to do, even in Sims 3 where your sim needed to max out their cooking skill in order to even learn that recipe. And the book of life, your sim need to get permission to write about their life first, and then present it to them just before they hit the floor.
And is why I have never bother to complete these in the game.
If I feel like saving a sim from dying, I just tell one of my sims to plead to Grim for that sim's life is all.
And sorry for the whole mix up above. I read death flower assumed that you were having a hard time getting one in the game.