I get an error in my gun when I unequip and equip it. How can I Fix This?

You never defined "Mouse" in the second function where line 20 resides.

my solution was to create a variable named "mouse" and set it to nil when you equip the gun where you define "Mouse" I redefine "mouse" to equal to "Mouse"

local maxAmmo = 8
local ammo = maxAmmo
local reloading = false
local player = game:GetService("Players").LocalPlayer
local playerGui = player:WaitForChild("PlayerGui")
local textLabel = playerGui:WaitForChild("AmmoDisplay"):FindFirstChild("AmmoText")
local mouse = nil

script.Parent.Equipped:Connect(function(Mouse)
    mouse = Mouse
    local function reload()
        reloading = true
        wait(1)
        ammo = maxAmmo
        reloading = false
    end

    script.Parent.Activated:Connect(function()
        if ammo > 0 and not reloading then
            ammo = ammo -1 
            script.Parent.gunShot:Play()
            if mouse.Target.Parent:FindFirstChild("Humanoid") then
            script.Parent.DealDamage:FireServer(Mouse.Target.Parent, 20)
            end
            elseif reloading == false then
            reload()
            script.Parent.gunShot:Stop()
        end

        while wait() do
            textLabel.Text = (ammo.. " / "..maxAmmo)
        end
    end)

    local input = game:GetService("UserInputService")
    input.InputBegan:Connect(function(key)
        if key.KeyCode == Enum.KeyCode.R and reloading == false and ammo ~=         maxAmmo then
            reload()
            textLabel.Text = "Out of ammo Press R to reload"
        end
    end)
end)

local Input = game:GetService("UserInputService") 
Input.InputBegan:Connect(function(key)
    if key.KeyCode == Enum.KeyCode.R and reloading == false and ammo ~=     maxAmmo then
        script.Parent.reload:Play()
    end
end)