How can i make this gamepass script work for speed and jump?

The problem is that the local script is executed after the player is added, therefor, PlayerAdded will not fire for you.

Luckily, you won't need to listen to PlayerAdded at all in your local script. Instead, when the user presses 'Left Shift', just check if that Player has the Gamepass, then change their walkspeed/jumppower accordingly.

local bought = game:GetService("MarketplaceService")

local speed = 6451920

--when we BEGIN pressing LeftShift
game:GetService("UserInputService").InputBegan:connect(function(input,gameprocesed)
	if input.KeyCode == Enum.KeyCode.LeftShift then --when we press Left Shift, then
		if bought:UserOwnsGamePassAsync(game.Players.LocalPlayer.UserId, speed) then --code block to check if the player has the GamePass!
			--if they bought the gamepass, then
			game.Players.LocalPlayer.Character.Humanoid.WalkSpeed = 60 --change our walkspeed
			game.Players.LocalPlayer.Character.Humanoid.JumpPower = 100 --change our movespeed
		end
	end
end)

--when we END pressing LeftShift
game:GetService("UserInputService").InputEnded:connect(function(input,gameprocesed)
	if input.KeyCode == Enum.KeyCode.LeftShift then --when we press Left Shift, then
		if bought:UserOwnsGamePassAsync(game.Players.LocalPlayer.UserId, speed) then --code block to check if the player has the GamePass!
			--if they bought the gamepass, then
			game.Players.LocalPlayer.Character.Humanoid.WalkSpeed = 16 --change our walkspeed
			game.Players.LocalPlayer.Character.Humanoid.JumpPower = 50 --change our movespeed
		end
	end
end)

I tried to preserve your code as best as possible. In Roblox, local scripts already know that the Local Player exists (they won't work without it), so you don't need to wait for it with PlayerAdded (I think that's where you are confused?)