My script has one error which I can't resolve

When I run this script, this error pops up.
[ERROR] lua/romester_fun_gun.lua.lua:7: attempt to index global ‘SWEP’ (a nil value)

  1. unknown - lua/romester_fun_gun.lua.lua:7

This is my code underneath

if SERVER then // This is where the init.lua stuff goes.
	//This makes sure clients download the file
	AddCSLuaFile ("shared.lua")
	//How heavy the SWep is
	SWEP.Weight = 3
	//Allow automatic switching to/from this weapon when weapons are picked up
	SWEP.AutoSwitchTo = false
	SWEP.AutoSwitchFrom = false
elseif CLIENT then // This is where the cl_init.lua stuff goes
	//The name of the SWep, as appears in the weapons tab in the spawn menu(Q Menu)
	SWEP.PrintName = "r0mester's fun gun"
	//Sets the position of the weapon in the switching menu 
	//(appears when you use the scroll wheel or keys 1-6 by default)
	SWEP.Slot = 4
	SWEP.SlotPos = 1
	//Sets drawing the ammuntion levels for this weapon
	SWEP.DrawAmmo = true
	//Sets the drawing of the crosshair when this weapon is deployed
	SWEP.DrawCrosshair = false
	//Ensures a clean looking notification when a chair is undone. How it works:
	//When you create an undo, you specify the ID:
	//		undo.Create("Some_Identity")
	//By creating an associated language, we can make the undo notification look better:
	//		language.Add("Undone_Some_Identity", "Some message...")
	language.Add("Undone_Thrown_SWEP_Entity","Undone Thrown SWEP Entity")
SWEP.Author = Romester
SWEP.Contact = ""
SWEP.Purpose = "Purely for Romester and fellow admins to use. Surprise included! Its my first gun by the way! :)"
SWEP.Instructions = "Press the LMB button and fire away."
//The category that you SWep will be shown in, in the Spawn (Q) Menu 
//(This can be anything, GMod will create the categories for you)
SWEP.Category = "Weapons"
SWEP.Spawnable = false -- Whether regular players can see it
SWEP.AdminSpawnable = true -- Whether Admins/Super Admins can see it
SWEP.ViewModel = "models/weapons/v_357.mdl" -- This is the model used for clients to see in first person.
SWEP.WorldModel = "models/weapons/w_357.mdl" -- This is the model shown to all other clients and in third-person.
//This determins how big each clip/magazine for the gun is. You can 
//set it to -1 to disable the ammo system, meaning primary ammo will 
//not be displayed and will not be affected.
SWEP.Primary.ClipSize = 5
//Bouncy bouncy :)
SWEP.Ricochet= true
//This sets the number of rounds in the clip when you first get the gun. Again it can be set to -1.
SWEP.Primary.DefaultClip = 10
//Obvious. Determines whether the primary fire is automatic. This should be true/false
SWEP.Primary.Automatic = false
//Sets the ammunition type the gun uses, see below for a list of types.
SWEP.Primary.Ammo = "none"
SWEP.Secondary.ClipSize = -1
SWEP.Secondary.DefaultClip = -1
SWEP.Secondary.Automatic = false
SWEP.Secondary.Ammo = "none"
//When the script loads, the sound ''Metal.SawbladeStick'' will be precached, 
//and a local variable with the sound name created.
local ShootSound = Sound("Metal.SawbladeStick")
function SWEP:Reload()
function SWEP:Think()
function SWEP:throw_attack (model_file)
	//Get an eye trace. This basically draws an invisible line from
	//the players eye. This SWep makes very little use of the trace, except to 
	//calculate the amount of force to apply to the object thrown.
	local tr = self.Owner:GetEyeTrace()
	//Play some noises/effects using the sound we precached earlier
	//We now exit if this function is not running serverside
	if (!SERVER) then return end
	//The next task is to create a physics prop based on the supplied model
	local ent = ents.Create("prop_physics")
	//Set the initial position and angles of the object. This might need some fine tuning;
	//but it seems to work for the models I have tried.
	ent:SetPos(self.Owner:EyePos() + (self.Owner:GetAimVector() * 16))
	//Now we need to get the physics object for our entity so we can apply a force to it
	local phys = ent:GetPhysicsObject()
	//Check if the physics object is valid. If not, remove the entity and stop the function
	if !(phys && IsValid(phys)) then ent:Remove() return end
	//Time to apply the force. My method for doing this was almost entirely empirical 
	//and it seems to work fairly intuitively with chairs.
	phys:ApplyForceCenter(self.Owner:GetAimVector():GetNormalized() *  math.pow(tr.HitPos:Length(), 3))
	//Now for the important part of adding the spawned objects to the undo and cleanup lists.
	cleanup.Add(self.Owner, "props", ent)
	undo.Create ("Thrown_SWEP_Entity")
		undo.AddEntity (ent)
		undo.SetPlayer (self.Owner)
//Throw an office chair on primary attack
function SWEP:PrimaryAttack()
	//Call the throw attack function, with the office chair model
//Throw a wooden chair on secondary attack
function SWEP:SecondaryAttack()
	//Call the throw attack function, this time with the wooden chair model

Any ideas on what I should do?

I just tried it and it works fine. I had some fun throwing chairs and father Grigori.
Script should be in addons/whatever/lua/weapons/whatever/shared.lua

Move the code to lua/weapons.

Moving to code to lua/weapons did remove the error.
But the gun doesn’t appear in the game.
I tried

But it said that the file could not be found.

Try give romester_fun_gun

Thanks everyone for helping out.