That is because “Left Arm” isn’t a property or anything like that, but a child. I also do wish people made more informed replies instead of just guessing, but oh well.
You might want to make variables for these arms since you’re repeating yourself unnecessarily
local char = script.Parent
local left_arm = char:WaitForChild("Left Arm") :: BasePart
local right_arm = char:WaitForChild("Right Arm") :: BasePart
local humanoid = char:WaitForChild("Humanoid")
-- you should already have the function defined
make(left_arm)
make(right_arm)
left_arm:Destroy()
humanoid:TakeDamage(10)
task.wait(math.random(5,20))
right_arm:Destroy()
humanoid:TakeDamage(10)