Title basically says it all. Put a CratesPlus key in a kit and it just makes it improperly defined.

EssentialsX does not (as far as I am aware) support data on items in kits that is specific to other plugins. You may want to use another kits plugin that explicitly supports the CratesPlus plugin.

Hmm 🤔, I'll look elsewhere. Would a plugin something like item on first join work?