I didn’t look at the tutorial, just at the file. Did you intend to have an animation demonstrating the katana leaving its sheath and the following problems? I see four actions, but only one of them seems to do anything, and that’s a walk cycle.
Looking at the rig, I can’t really see exactly what you’re trying to do-- I see a long chain of bones with either copy transforms constraints or parenting, eventually reaching michin_espada_hand_R, which has no connection to anything. And that’s mirrored for the sheath.
(And personally, I’m just not interested in watching 16 mins of some random youtube tutorial.)
So, I’m not sure exactly what your goal is, or where you were getting stuck. The constraints you currently have on the rig aren’t enough to get the sword in the scabbard.
But, I’m pretty sure whatever you’re doing, the answer lies in the use of the “apply visual transforms” operation combined with animating the influence of constraints. (An alternative to “apply visual transforms” is keyframing visuals, but I’ve had bad luck with that, possibly just in earlier versions, and I just use apply visual transforms and key manually.)
For example, if I want the sword to move with the scabbard, I might create a “copy transforms” constraint targeting a scabbard bone. Then I would go to the last frame when it moved with the scabbard, keyframe constraint influence and loc/rot/scale; then, advance one frame, apply visual transform, set constraint influence to zero, and keyframe constraint influence and loc/rot/scale.
It’s well worth figuring out what “apply visual transforms” does, but the best way to figure that out is to just play with it. It’s a very useful tool that solves all sorts of animation problems.
You could check out https://blender.stackexchange.com/questions/210021/how-to-unattach-an-object-and-reattach-it-somewhere-else-in-an-animation/210024#210024 where I answered a similar question. That’s using a child-of constraint, but if you squint, you should be able to see that it’s the same problem that you have.
(Just grateful that it’s not actually a katana, but a straight sword. Curve of the katana complicates the draw…)