How can I change this Walkspeed gamepass script for it to work?

Use UserOwnsGamePassAsync to check if the player has a pass:

local gamepassId= YOUR GAMEPASS's ID

game.Players.PlayerAdded:Connect(function(player)
	player.CharacterAdded:Connect(function()
		local hasGamepass = game:GetService("MarketplaceService"):UserOwnsGamePassAsync(player.UserId, gamepassId) 
		if hasGamepass then
   			plr.Character:FindFirstChild("Humanoid").WalkSpeed = 200
		end
	end)
end)

#Preventing speed hackers: Keep in mind that hackers can change their characters physics which means they can change their speed. You should have some sort of check to see if they DON'T own the gamepass and they're faster than normal.

The script would look something like this:

local gamepassId = YOUR GAMEPASS's ID

local previousPositions = {}
local MAX_SPEED = 25  

function checkMovement(player)
	local character = player.Character
		
	if character then
		local hasGamepass = game:GetService("MarketplaceService"):UserOwnsGamePassAsync(player.UserId, gamepassId) 
		local previousPosition = previousPositions[player.Name]

		if not hasGamepass and previousPosition then
			local distanceTraveled = (Vector3.new(previousPosition.X,0,previousPosition.Z) - Vector3.new(character.HumanoidRootPart.Position.X,0,character.HumanoidRootPart.Position.Z)).magnitude
			print(distanceTraveled)
			if distanceTraveled > MAX_SPEED then
				player.Character.HumanoidRootPart.Position = previousPositions[player.Name]
			end
		end

		previousPositions[player.Name] = player.Character.HumanoidRootPart.Position
	end
end

while wait(1) do
	for _, player in pairs(game.Players:GetPlayers()) do
		checkMovement(player)
	end
end 


--remove player from previousPositions table when they leave the game
game.PlayerRemoving:Connect(function(player)
	previousPositions[player.Name] = nil
end)