Problem with adding IK/FK snap into my custom rig

So I found this nice tutorial for a quick and easy IK/FK snapping. I tried to apply it to my custom Minecraft rig. I manage to implement this feature into the arm, but I still have an issue with the leg. Apparently, it has something to do with how the leg was designed. I’m not the original creator of the rig and I don’t know much about rigging, so it’s kinda hard for me to do this by myself. I hope there’s someone who can help me with implementing this IK/FK snapping feature into my rig and hopefully learn something.

Here’s the link to the tutorial I followed for my IK/FK snap:

And here’s the link for the rig I currently working on: