Why cant i access coregui?

local script:

if game.CoreGui:FindFirstChildOfClass("LocalScript") then
	game.ReplicatedStorage["anti exploit"]:FireServer()
end

With CoreGui you are really limited with what you can do.

CoreGui does not have allow you to perform a lot of operations on it, as ROBLOX has disabled them due to keeping consistency on all their games.

However this doesn't mean you can't access CoreGui, but you can only perform a few limited functions on it.

You can enable/disable certain aspects of the GUI, as seen here (e.g. Backpack, the chat window):

https://developer.roblox.com/en-us/api-reference/function/StarterGui/SetCoreGuiEnabled

But for other aspects you can only set them to be transparent or not:

https://developer.roblox.com/en-us/api-reference/function/PlayerGui/SetTopbarTransparency

However with what you're trying to do ROBLOX does not allow you to look/change for anything inside of the CoreGui.