How do I make my fly script require a gamepass to work?

You can use MarketplaceService for this. First, upload a gamepass and then get its ID number(the number is in the link of the gamepass). Then you would want to check if the player owns the gamepass using :UserOwnsGamePassAsync().

Here is what to do.

local MarketplaceService = game:GetService("MarketplaceService")

local player = game.Players.LocalPlayer
local char = player.Character or player.CharacterAdded:Wait()

--Check If the Player Owns the Gamepass
local gamepassId = 123456789 --Insert your gamepass ID here.
----If player doesn't own the gamepass, stop the script
if not MarketplaceService:UserOwnsGamePassAsync(player.UserId, gamepassId) then return end

local cam = workspace.CurrentCamera
local uis = game:GetService("UserInputService")

local idleAnim = char:WaitForChild("Humanoid"):LoadAnimation(script:WaitForChild("FlyIdle"))
local forwardAnim = char:WaitForChild("Humanoid"):LoadAnimation(script:WaitForChild("FlyForward"))


local wPressed = false
local sPressed = false
local aPressed = false
local dPressed = false

local flying = false
uis.InputBegan:Connect(function(key, chat)
	if chat then return end
	if key.KeyCode == Enum.KeyCode.R then
		if flying then --Stop Flying
			flying = false
			char.Animate.Disabled = false
			idleAnim:Stop()
			forwardAnim:Stop()

		else --Start Flying
			flying = true

			char.Animate.Disabled = true
			idleAnim:Play()

			local bv = Instance.new("BodyVelocity", char.PrimaryPart)
			bv.MaxForce = Vector3.new(math.huge,math.huge,math.huge)
			bv.Velocity = Vector3.new(0,0,0)
			bv.Name = "FlightForce"

			repeat wait(0.1) until flying == false
			bv:Destroy()
		end
	end

	if key.KeyCode == Enum.KeyCode.W then
		wPressed = true
	elseif key.KeyCode == Enum.KeyCode.S then
		sPressed = true
	elseif key.KeyCode == Enum.KeyCode.A then
		aPressed = true
	elseif key.KeyCode == Enum.KeyCode.D then
		dPressed = true
	end
end)

uis.InputEnded:Connect(function(key)    
	if key.KeyCode == Enum.KeyCode.W then
		wPressed = false
	elseif key.KeyCode == Enum.KeyCode.S then
		sPressed = false
	elseif key.KeyCode == Enum.KeyCode.A then
		aPressed = false
	elseif key.KeyCode == Enum.KeyCode.D then
		dPressed = false
	end
end)

while wait() do
	if flying then
		char.PrimaryPart:FindFirstChild("FlightForce").Velocity = Vector3.new(0,0,0)
		forwardAnim:Stop()

		if wPressed then
			char.PrimaryPart:FindFirstChild("FlightForce").Velocity = cam.CFrame.LookVector * 100
			forwardAnim:Play()
		end
		if sPressed then
			char.PrimaryPart:FindFirstChild("FlightForce").Velocity = cam.CFrame.LookVector * -100
			forwardAnim:Play()
		end
		if aPressed then
			char.PrimaryPart:FindFirstChild("FlightForce").Velocity = cam.CFrame.RightVector * -100
			forwardAnim:Play()
		end
		if dPressed then
			char.PrimaryPart:FindFirstChild("FlightForce").Velocity = cam.CFrame.RightVector * 100
			forwardAnim:Play()
		end
	else
		wait(1)
	end
end

Since I assume you are using a local script, the script checks if the player owns the pass, if the player doesn't own the pass, the script will end and nothing will happen. But if the player owns the gamepass, the script will continue as normal.