My door keeps spinning, and won't stop at the 90 degree angle mark. What's wrong with it?

Lerp stands for Linear intERPolation. CFrame:lerp(new, d) is defined as a new CFrame that is d of the distance between the current CFrame and new. d is usually between 0 and 1 but it can be larger to go past new. Heartbeat event passes the time since the last Heartbeat in seconds. It does not give you the time since you connected your listener to the event. In addition, finish is constantly being recalculated to be 90 degrees ahead of where the door is now. I've fixed all of these problems here.

local debounce = true
local Open = false

script.Parent.Event.OnServerEvent:Connect(function(Player)
	local Mag = (script.Parent.Center.Position-Player.Character.HumanoidRootPart.Position).magnitude
	if Mag <= script.Parent.Range.Value then
		if debounce then
			debounce = false
			if Open == false then
				local ellapedTime = 0
				local finish = script.Parent.PrimaryPart.CFrame * CFrame.Angles(0,math.rad(90),0)
				local connection -- this syntax is necessary
				connection = game:GetService('RunService').Heartbeat:Connect(function(lastBeat)
					ellapsedTime = ellapsedTime + lastBeat
					local d = ellapsedTime / TIME_TO_OPEN -- how long you want it to take to open
					local cfm = script.Parent.PrimaryPart.CFrame:lerp(finish, math.clamp(d, 0, 1))
					script.Parent:SetPrimaryPartCFrame(cfm)
					if d >= 1 then 
						connection:Disconnect()
					end
				end)
				Open = true
			else
				local ellapedTime = 0
				local finish = script.Parent.PrimaryPart.CFrame * CFrame.Angles(0,-math.rad(90),0)
				local connection -- this syntax is necessary
				connection = game:GetService('RunService').Heartbeat:Connect(function(lastBeat)
					ellapsedTime = ellapsedTime + lastBeat
					local d = ellapsedTime / TIME_TO_CLOSE -- how long you want it to take to close
					local cfm = script.Parent.PrimaryPart.CFrame:lerp(finish, math.clamp(d, 0, 1))
					script.Parent:SetPrimaryPartCFrame(cfm)
					if d >= 1 then 
						connection:Disconnect()
					end
				end)
				Open = false
			end
			debounce = true
		end
	end
end)