Attempt to index nil with Humanoid?

Your code is likely being called before the player's character is created. If the player's character has not yet been created, it will be equal to nil.

There are a few possible solutions to this issue. These are two solutions to the issue:

repeat
    wait()
until player.Character

and

if not player.Character then
    player.CharacterAdded:Wait()
end

Inserted into your code it would looks something like this:

local ExcaliburAnimation = script["Excalibur Animation"]
local player = game.Players.LocalPlayer
--if not player.Character then
--  player.CharacterAdded:Wait()
--end
-- ^^ that would work when un-commented
local char = player.Character
repeat
    char = player.Character
    wait()
until player.Character ~= nil
local humanoid = char.Humanoid

local ExcaliburAnimationTrack = humanoid:LoadAnimation(ExcaliburAnimation)

ExcaliburAnimationTrack:Play()

Sometimes the character will load after the script has ran. It says humanoid index nil so I assume that character is nil. You should add a script that waits for the character to load in.

if not char then
    char = player.CharacterAdded:Wait()
end

This should solve it if you insert it between 3 and 4