how to fix walk speed gamepass?

First off, Walkspeed should be WalkSpeed. Second, you cant confirm that the player is buying it unless you know the player. Hope this works :)

local id = 14645123

BuyEvent.OnServerEvent:Connect(function(Player)
    game:GetService("MarketplaceService").PromptGamePassPurchaseFinished:Connect(function(plr,ido,purchased)
        if purchased and id == ido and plr == Player then
            plr.Character.Humanoid.WalkSpeed = 40
        end
    end)
end)

game.Players.PlayerAdded:Connect(function(plr)
    if game:GetService("MarketplaceService"):UserOwnsGamePassAsync(plr.UserId,id) then                       
        game.Workspace:WaitForChild(plr.Name):WaitForChild("Humanoid").WalkSpeed = 40
    end
end)

Howdy, there are two possibilities that your script isn't working

1. You are setting both WalkSpeed's to 40, purchasing it or not.

2. It is waiting for the plr or the humanoid but can't find it.

Do you own the gamepass?