First thing I do when debugging rigs, go to pose mode, select all bones, altR and altG to remove transforms. Do it again, and if bones jump around as they do here it’s not a good sign. It means IK probs, and the problem here is the pole target has been designated as the foot. Except the foot is the child of the bone with the IK constraint on it, therefore it controls the bone that controls it… this is called a cyclic dependency and it causes that flickering because Blender can’t resolve the final location for that bone.
The solution is to have pole targets that are completely independent of any bone in the chain. Usually they ar parented to a ROOT bone, but the tutorial you were foliowing (didn’t watch it all, sorry) might not have a root. In any case what you need to do is ADD two more bones to your rig if you want pole targets. I did that in the file I’m reposting but you should do it yourself just for practice…
Delete the pole target field in the IK. Do this first, so Blender does not crash, although Blender handles these errors with a great deal of grace.
make a new bone in the same armature, and position it. Make sure it’s not childed to any other bone.
Designate this as the new pole target.
Do one for the elbow too.
I did note that the video parented the foot bone to the IK target, this means that you have to be careful how you drag the ik target because the foot will stretch the mesh as it stays with. Not sure how he’s going to go on and fix that, but I prefer to do it the way I have in the attached file.
He also parented the arms to the hip which seems awkward, and I changed that too, and although your Bear seems rather too limber in the belly, as in too many bones, that’s just a minor thing - gymnastic bear maybe?