Out of all the complex solutions given on multiple pages of devforum, this is what worked, respect
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Oh, i didn’t mean World Model Class name, i was just calling the Model that will be in the Workspace.
iirc I had the same issue, the main problem is that something isn’t a descendant of the workspace/worldmodel.
I had to add some sort of repeat task.wait() checking if the humanoid and other relevant things are ancestors of the workspace/worldmodel.
I got his with this too. Very frustrating.
This is still happening in 2023. I fixed this by creating an Animator object in the player’s Humanoid with a script placed in StarterCharacterScripts, then updating all my other scripts that load animations to wait for the Animator object inside the Humanoid, and call :LoadAnimation() on it, instead of the Humanoid object. It probably has something to do with the fact that calling :LoadAnimation() on a Humanoid or AnimationController has been deprecated for almost 3 years now (And I only found out about this recently, lol).
I had a very similar problem like this as well. I had to use very weird code in order to fix it, I made it so that the character was added to a scope and the humanoid in a equip function and played the track by loading it in while equiping it. But to stop the animation, i added an anonymous unequip function to the equipped function, It works but there’s probably a better way to do it. This is what i mean:
function OnEquipped()
local newChar = Players.LocalPlayer.Character or Players.LocalPlayer.CharacterAdded:Wait()
local hum = newChar:WaitForChild("Humanoid")
local track = hum.Animator:LoadAnimation(anim)
track:Play()
Tool.Unequipped:Connect(function()
track:Stop()
end)
end
yeah i fixed this by waiting until the character was inside of workspace
you can use the WorldModel instance
YO, THIS ACTUALLY WORKED, EVERYONE JUST PUT
wait() or task.wait()
Before all your lines of script it works, trust 
Thank you, @BuilderBird_Dev!!
Couldn’t this result in infinite yeild?
For the people who might encounter this, here is a simple explanation of what causes this and how to fix it
Explanation
This error occurs when you attempt to animate a humanoid or animator that isn’t a child object of game. This is a common issue when loading objects into the world and trying to directly attach an animation into them. Here’s an example on how to cause this error:
local animation = Instance.new("Animation")
animation.AnimationId = "http://www.roblox.com/asset/?id=507770818"
local npc = workspace.Rig
local humanoid = npc.Humanoid
npc.Parent = nil
humanoid:LoadAnimation(animation)
Fix
There’s 2 ways to go about this. If the character indexing is under your power make sure to Parent it BEFORE loading the animations. If it isn’t under your power you can do something along the lines of:
local ANIMATION_TIMEOUT: number = 1.5
local function loadAnimation()
local timer: number = 0
while not humanoid:IsDescendantOf(game) do -- Check if the animator is in the game
timer += task.wait()
if timer > ANIMATION_TIMEOUT then -- timeout
return
end
end
humanoid:LoadAnimation(animation)
end
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You have to put the animated object as a descendant of workspace. That’s it
Why does this work, but seriously thank you 
In my opinion, the solution was to make LoadAnimation function run late with wait() because the execution of another function had to be completed for the function to run.
It’s 2024 and I’m STILL getting this problem. Why does Roblox have to be this buggy? Why can’t they fix these problems? Like come on man.
Thank you this fixed the error for me when I set the model’s parent to workspace before loading animations!
it works because plr wait iss just waiting for the player to be added instead of waiting for the character to be added or just rolling with the incomplete one
Well, no. Both of those have their issues. The former will yield forever if the script runs after the character loads, and the latter will give you a dead character if the script runs after you die but immediately before you respawn. What you really want is:
local Player = game:GetService("Players").LocalPlayer
local function GetCharacter()
local Char = Player.Character
return if Char and Char.Parent then Char else Player.CharacterAdded:Wait()
end
Apologies if im about 2 years late, i had the same issue although all i had to do is add task.wait(2) above the animation line and it worked