If you’re talking about quest #5, this is what I have written in the Sub Quests page:
The location of the medicinal plant may be entirely random. I found it on the 2nd floor of Resapp Cave, but you may have to go to both locations in order to get one.
If you’ve completed Quest 04, you can equip the Encounter Ring B to either reduce or eliminate the random encounters while you’re searching.
Forgive me, because it’s been a long time since I played this game – but I think in order to complete this quest, you only need 1 medicinal plant. (It’s just that the plant appears in one of two locations, so if you don’t find it in the first one you need to check the second.)
Please let me know if that’s incorrect, or if the information I’ve given is not helpful.