I'm guessing this is a very common rookie mistake but I keep getting these errors as I try to build a custom hero:

https://imgur.com/a/Rm6ovAX

I have a rough idea of what might be wrong? And I just want to try all options before I have to redo everything from scratch.
For some context, I've built this 'custom animation' from modifiying the skeleton of the Jester. I deleted a few bones here and there, and It seems I deleted the fx--- bones. Some at least.
I've added them back, which reduced the error only to the idle skeleton.
Any tips, or suggestions on how to procede?