:FindFirstChild can return nil. That's when the child doesn't exist.

Say there's an object in the workspace called "Apple" and I want to change its color

local apple = workspace.Apple
apple.BrickColor = BrickColor.red()

If there isn't an object named "Apple", then when I do workspace.Apple, I will get an error saying there is no such thing.

I could instead use :FindFirstChild:

local apple = workspace:FindFirstChild("Apple")

Now, I won't get any error. But apple is nil, so if I try to do something to it:

apple.BrickColor = BrickColor.red()

I will get an error.

If you're going to use :FindFirstChild, you have to have a plan of what to do when the object doens't exist -- otherwise you should just use . since it's a lot shorter and clearer.

if not apple then -- if apple ~= nil then
	apple.BrickColor = BrickColor.new()
else
	-- I can't do anything.
end

WaitForChild

In your case, it looks like you're expecting there to be something there. In that case, you should probably use :WaitForChild.

:WaitForChild is similar to :FindFirstChild, except that it is guaranteed to never return nil -- it does this by waiting until there is an object that it can return.

If :WaitForChild isn't a solution, probably you are either looking in the wrong place (wrong script.Parent, etc) or the object is actually named something else (capitalization counts, etc)