Apologies for my very late response. Apparently Humanoid:LoadAnimation has been depreciated in favor of Animator:LoadAnimation. Not sure if it will fix your error if you change it, but try this:
local animator = humanoid:WaitForChild("Animator")
local IdleAnimation = animator:LoadAnimation(IdleAnimId)
local ShootAnimation = animator:LoadAnimation(ShootAnimId)
See the announcement here for more information.