How to use PromptGamePassPurchaseFinished?

You're treating PromptGamePassPurchaseFinished as if it were a function that returns a boolean, determining if the purchase was finished or not.

It is an event; you listen for it.

The listeners of this event get three arguments:

1. the player who was prompted
2. the game pass ID
3. boolean if they purchased the pass or not.

But instead of connecting a function, we'll wait for it via the :Wait() method.

:Wait() returns what would be given as arguments to the listeners.

This code can also definitely be cleaned up. MP is a constant as the value won't change, so why declare the variable each time the part is touched?

local Players,
MarketplaceService = 
game:GetService("Players"),
game:GetService("MarketplaceService");
local PASS_ID = 1087508; -- constant number 

local function protectedOwnsPass(client, passId)
    -- Functions labeled as 'Async' make web requests and may fail
    local success, result = pcall(
        MarketplaceService.UserOwnsGamePassAsync,
        MarketplaceService,
        client.UserId,
        passId
    );

    if (success) then
        return result; -- successful web request
    end
    warn("Error whilst web request:", result);
end

script.Parent.Touched:Connect(function(part)
    local client = Players:GetPlayerFromCharacter(part.Parent);

    if (client and protectedOwnsPass(client, PASS_ID)) then
        return; -- they own the game pass, no more needs to be done
    end
    local humanoid = client.Character.Humanoid
    local prevSpeed = humanoid.WalkSpeed
    local bought = select(3, MarketplaceService.PromptGamePassPurchaseFinished:Wait());

    if (not bought) then -- they didn't buy it
        humanoid:TakeDamage(humanoid.Health);
        -- basically just kills them
    else
        humanoid.WalkSpeed = prevSpeed;
        -- they bought it
    end
end);

Some things that were used, but not explained

• select selects a certain argument, and everything after it. So select(2, 'a', 'b', 'c', 'd') evaluates to 'b', 'c', 'd' since the second argument is selected, and everything else after it is selected also. If the first argument is a '#', it returns how many arguments were passed (excluding the first, which is what to select).

• pcall (short for protected call), takes a function, and calls it in protected mode, so any runtime errors will not be propagated. Important for if your function makes web requests, or is just prone to error in general. The first result of this function is a boolean, true if the function was successfully called without errors, false otherwise. Everything after the first return value is what the function returns, but only if there wasn't an error. If there was an error, the first return value is false, and the second return value is the error message.