“Infinite yield possible on WaitForChild” indicates that the part you are trying to find doesn’t exist yet. By “Infinite yield possible”, this means in your script, there is a chance that it would wait infinitely. So anything under WaitForChild’s thread will never run until the instance is found.
This is just a warning, not an error. A warning doesn’t do anything. It just shows the output that there seems to be something wrong with your script. An error(a red circle with an exclamation mark(!)) stops the current thread in your script.
If you don’t want to do this, you can just do the following
local instance = CoreGui:FindFirstChild("TopBarApp")
-- Your code here
if not instance then
repeat
task.wait()
until instance
end
-- Your code here