Trying to get my battle system working but getting errors?

[DiscoInFEARNo]

Hey I'm working through trying to get certain mechanics working in my game, namely the LeTBS but I get this error?



The screenshot shows the error I am getting along with the deveoper tools open, If anyone can help me make sense of this it would be greatly appreciated.

 

[Negi-Mox]

Maybe this can help:
https://lecodemv.github.io/leTBS/