Fire is not a valid member of RBXScriptSignal??

I want to be able to know when one of my core-scripts stops responding. I have this code in one script:

(MAIN SCRIPT)

-- Error State

script.Error_State.Event:connect(function(x)
if x == "Check" then
script.Error_State.Event:Fire("Return")
end
end)

And this is the code for the other script

local Alive = true
local ErrorState = false

wait(5)

game:GetService('RunService').Heartbeat:connect(function()
if ErrorState == false then
Alive = false
script.Parent.Error_State:Fire("Check")
wait(0.1)
if Alive == false and ErrorState == false then
game.ReplicatedStorage.Abscond_Materials.Abscond_Event:FireAllClients("Alert","The Abscond System crashed unexpectedly(script stopped responding). Please wait while the system tries to recover. Your in-game experience may be interrupted.","The Abscond System.")
ErrorState = true
end
end

if Alive == true and ErrorState == true then 
game.ReplicatedStorage.Abscond_Materials.Abscond_Event:FireAllClients("Notification","The Abscond System crashed unexpectedly and successfully recovered. Thank you for your patience.","The Abscond System.")
ErrorState = true
end
end)

script.Parent.Error_State.Event:connect(function(x)
if x == "Return" then 
Alive = true
end
end)

I've tried to add waits, etc, but that doesn't seem to be working.

Whenever the MainScript tries to fire the event it says "Fire is not a valid member of RBXScriptSignal" - what does this mean? How do I fix it?

It means that the .Event you have is of the class RBXScriptSignal. If you check out that wiki link, you will notice that there is no such thing as "Fire" in it -- only "Wait" and "Connect" ("connect" is deprecated, so you should stop using it).

If Error_State is a BindableEvent, simply call ":Fire" on it: script.Error_State:Fire("Return")

Do note that just because your script responds to this BindableEvent doesn't mean that it's still working. Since events start their listeners in new coroutines every time, it will likely always fire the "Return" value (unless it died before it got to that point). Instead, if your script uses a while true do loop, you may want it to continually adjust an IntValue and have your "has it crashed?" script assume that if the IntValue stops updating for more than some period of time (ex a fraction of a second or whatever is reasonable), that it has crashed.