maybe neither but the code you used or the plugin in the wrong order or
started from a existed savefile after adding those plugin or made changes to it.
you need exactly tell what you did before you get this error.
Well I just started testing encounter recently and don't know what plugin might have caused this.
I add every YEP plugins just like their instruction.
The ID of the enemy is [4] with troop variant ranging from [8]-[11].
I tried copying Slime[3] enemies to ID[10], changed the graphic to Bat, then adding the enemy to a new Troops[14] but I don't encounter the problem regardless of where I place it or how much enemy present.
I Also tried making a new Troop[15] with the original Bat[4] enemies but it work just fine.
I can avoid the Troop ID[8-11] as workaround I think. But idk where else the bug will be present.
What is your version of MV? What is the version of your core files? To find out the second, you will need to go into your project's js folder, open the rpg_core.js file in an editor, and look for the version number on the first few lines. If you don't see a version number, it means you're on 1.0.0
You need to check both, because you could have an old rpg_core version even if you've got the latest editor version.
All of the RPG_ files are the latest version, 1.6.2.
EDIT: I SOLVE IT!
So I'm dumb and accidentally added Enemy ID that I use as separator on the database. It doesn't have any graphic so it didn't show anything in the Troop editor.
Thanks ShadowDragon and Shaz for helping.
Without you guys I would not have any idea to check Troop.json.
After comparing the code from another troop, I realized there exist an unwanted enemy.