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?