i have fixed it…
There’s a link to this Unreal document that explains it: https://docs.unrealengine.com/en-US/Programming/Plugins/index.html
The relevant bit is: When you install it as an engine plugin, you place it in /[UE4 Root]/Engine/Plugins/KawaiiPhysics/, while to install it as a game plugin you place it in [Project Root]/Plugins/KawaiiPhysics/