FireServer can only be called from the client? Please help!

I'm working on a simulator and this is a script for a UpgradeGui. On line 7 is where the error is. If someone helps it would be really helpful!

Thanks, notreallysupernova

--[[ First, put a RemoteEvent in Replicated Storage before doing any of this.
	]]
-- Client/Local Script:
local remoteEvent = game:GetService("ReplicatedStorage"):WaitForChild("RemoteEvent")

script.Parent.MouseButton1Down:Connect(function()
	remoteEvent:FireServer() -- This is the error
end)

-- Server/Normal Script:
local remoteEvent = game:GetService("ReplicatedStorage"):WaitForChild("RemoteEvent")
local OwnUpgrade = game.ReplicatedStorage.Stats.OwnMoreCars
local CarRespawn = game.ReplicatedStorage.Stats.CarRespawn
local datastore = game:GetService("DataStoreService")
local ds1 = datastore:GetDataStore("OwnMoreCarsSaver")
local players = game:GetService("Players")

players.PlayerAdded:connect(function(player)
	OwnUpgrade.Value = ds1:GetAsync(player.UserId) or false
	ds1:SetAsync(player.UserId, OwnUpgrade.Value)
end)

if OwnUpgrade.Value == true then
	CarRespawn.Value = CarRespawn.Value - 1
	OwnUpgrade.Value = true
	print("Owned Upgrade")
	
	script.Parent:Destroy()
end

local player = remoteEvent.OnServerEvent:Wait()
local leaderstats = player:WaitForChild("leaderstats",7)
local Money = leaderstats:WaitForChild("Money")




local price = 300
if Money.Value >= price then
	Money.Value -= price
	print("Purchased")
	CarRespawn.Value = CarRespawn.Value - 1
	OwnUpgrade.Value = true
	
	ds1:SetAsync(player.UserId, OwnUpgrade.Value)
	script.Parent:Destroy()
	
end

The FireServer function can only be called on upon a localscript. Server scripts or normal scripts use the FireClient and FireAllClients function. The fire server is just really called as from a remote event and so the client can send data to the server that the remote has been fired.