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.