FireServer is not a valid member of RemoteEvent?

You misspelt FireServer as FireSever. There is still an issue with your script.

--script
local famas = game.ServerStorage.FAMAS
local scarL = game.ServerStorage["SCAR-L"]
local famasClone = famas:Clone()
local scarLClone = scarL:Clone()
local remote = Instance.new("RemoteEvent")
remote.Parent = game.ReplicatedStorage
remote.Name = "WeaponEvent"

remote.OnServerEvent:Connect(function(plr)
    if famasClone.Parent ~= plr.Backpack then
        famasClone.Parent = plr.Backpack
    end
    if scarLClone.Parent ~= plr.Backpack then
        scarLClone.Parent = plr.Backpack
    end
end)

This script would only clone once, and say a different player fires the event, the gun will be taken away from whoever has it. If this is the desired behaviour, keep it. If not, you should clone the weapon inside the event listener. Not outside of it.

remote.OnServerEvent:Connect(function(client) -- use a better parameter than plr. player is better too
	if not client.Backpack:FindFirstChild("FAMAS") and not client.Character:FindFirstChild("FAMAS") then
		famas:Clone().Parent = client.Backpack
	end

	if not client.Backpack:FindFirstChild("SCAR-L") and not client.Character:FindFirstChild("SCAR-L") then
		scarL:Clone().Parent = client.Backpack
	end
end)

Notice the additional check on the character. When you equip a tool it is parented to your character. So they can't give themselves as many tools as they want.