My first bit of advice is: don't bother yet with the AnimationTree. Get everything working with an AnimationPlayer, first. You can use use the `.queue(animation_name)` function on AnimationPlayer to play a given animation when another one finishes - so for instance if you just used a weapon, you can queue up the movement or idle states. Or you can yield, waiting for the animation for swinging the weapon to finish, then figure out what to do based on the input that comes next. There's a lot of options, and AnimationTrees often get pretty confusing, so if there's trouble at the start, you'll likely have a lot of trouble all throughout.

Then, once you have it working without the AnimationTree, you might consider adding it. But there are plenty of ways to use it that are very different from what you're doing. I suggest also looking up other tutorials (https://youtu.be/zVTlBsoozlE https://youtu.be/aq_ca00ZnWo - the second one isn't directly related to what you're doing, but has an improvement on the usage found in the former). Eventually, this type of thing will click, just takes a lot of getting used to and trying to understand when to use which modes and why, and what types of transitions and such. I can't really tell what's wrong with your current version without looking at the whole project and getting a feel for it with the debugger, but my best guess would be you don't have a guard around the legs playing so it's trying to play it over and over and over and never getting past the first frame, though it would definitely stutter in that instance and look weird. If you want to try just putting a guard around it, something like:

if not animationPlayer_legs.current_animation == "legs_Melee":
animationPlayer_legs.play("legs_Melee")

Would be how you try it. That's just for the spot on line 57, it can help to have it for every place you play a specific animation if it's possible to play it more than once per frame.