Why am I getting an error in the PlayerId?

Howdy!

MarketplaceService.ProcessReceipt is a callback, not an event. It's the same as having a RemoteFunction and setting the function to fire. For something like that'd, you write something like:

game.ReplicatedStorage.RemoteFunction.OnServerInvoke = MyFunction

That's the same thing here. Replace your last line in the script with what I have below.

msSERVICE.ProcessReceipt = proccessedReceipt

Edit:

So, I think the if-then statements had a misplaced end on it. Also, I'm confused why you're specifying a specific PurchaseId. Did you mean to look for a specific UserId? Also, I think you should have a statement for when neither of the PurchaseId's are what you're looking for. Regardless, I'm going to try to stick close to what you had originally. If you meant to look for UserId instead of a receipt number, I can help with that.

Try what I got on below and customize it to your liking. It'll check if the player is in-game and if their receipt number matches the correct receipt numbers listed. If the player is in-game and their receipt doesn't match any of the predetermined numbers, it'll warn in the output logs.

local function proccessedReceipt(receiptInfo) 
    local player = players:GetPlayerByUserId(receiptInfo.PlayerId) 
    if not player then 
        return Enum.ProductPurchaseDecision.NotProcessedYet
    else
        if receiptInfo.PurchaseId == 1149873023 then 
            warn("a") 
            local Mcopy = minigun1:Clone() 
            local char = game.Workspace:FindFirstChild(player.Name) 
            Mcopy.Parent = player.Backpack 
            warn("b") end 
            return Enum.ProductPurchaseDecision.PurchaseGranted 
        elseif receiptInfo.PurchaseId == 1149873432 then 
            warn("c") 
            local m2COPY = minigun2:Clone() 
            local char = game.Workspace:FindFirstChild(player.Name) 
            m2COPY.Parent = player.Backpack warn("d")
        else
            warn("This purchase has an incorrect receipt number.")
        end
    end
end 

msSERVICE.ProcessReceipt = proccessedReceipt

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Be sure to check out the Roblox API Documentation as well for additional reference.