How do I launch a cannon ball from my cannon using a GUI button?

SRRY FOR ANY TYPOS

Firstly you have used :connect which is deprecated (use :Connect). Secondly, you have done this :

game.StarterGui.seatgui.Frame.switch1.MouseButton1Click:connect(fire)

However, before this line of code you haven't even made the function yet. Put this line of code after you've made the function.

Thirdly, with the MouseButton1Click event, it doesn't give you the player who clicked(it gives you nothing). It would probably be better to use

local player = game:GetService("Players").LocalPlayer

or

local player = game.Players.LocalPlayer

to get the player.

Use this script and see if it works (cant guarantee that since, i haven't checked the whole code) Also, make sure this is in a LocalScript, and is in the button that is being clicked.

local gunBarrelOne = workspace.MediumBoat.cannonRR.GunBarrel.One
local player = game:GetService("Players").LocalPlayer --gets the player

local debounce = false
local gunOne = true

local cannonBall = Instance.new("Part")
cannonBall.Size = Vector3.new(1,1,1)
cannonBall.BrickColor = BrickColor.new(26) 
cannonBall.Shape = 0
cannonBall.BottomSurface = 0
cannonBall.TopSurface = 0
cannonBall.Name = "Cannon Shot"
cannonBall.Elasticity = .1
cannonBall.Reflectance = 0
cannonBall.Friction = 0


function fire()

    local sound = workspace.MediumBoat.cannonRR.Switch:findFirstChild("GunSound")
    if sound == nil then
        sound = Instance.new("Sound")
        sound.Name = "GunSound"
        sound.SoundId = "http://www.roblox.com/asset?id=2101148"
        sound.Volume = 1
        sound.Parent = workspace.MediumBoat.cannonRR.Switch
    end
    sound:play()


    local missile = Instance.new("Part")

    local barrel

    if gunOne == true then
        barrel = gunBarrelOne
        gunOne = true

    end

    local spawnPos = barrel.CFrame * Vector3.new(6, 0, 0)

    local dx = math.random(0,0)
    local dy = math.random(0,0)
    local dz = math.random(30,30)
    local mag = math.random(200,200)

    local v = barrel.CFrame:vectorToWorldSpace(Vector3.new(mag + dx,dy,dz))

    local missile = cannonBall:clone()

    missile.Position = spawnPos
    missile.Velocity = v



    local new_script =workspace.MediumBoat.cannonRR.Switch.CannonBall:clone()
    new_script.Disabled = false
    new_script.Parent = missile

    local creator_tag = Instance.new("ObjectValue")
    creator_tag.Value = player
    creator_tag.Name = "creator"
    creator_tag.Parent = missile

    missile.Parent = game.Workspace

end
script.Parent.MouseButton1Click:Connect(fire)
 --connect the function to the event after we've made it

This is due to a common mistake. Some people use StarterGui to get the player's gui, right? WRONG, you need to do

game.Players.LocalPlayer.PlayerGui

Also, try to use non-deprecated stuff as deprecated stuff may get removed at any time without notice.

Hope this helps!