I'll explain the code first.

The client sends a remote event to the server containing the string value for an ImageID and another string value called CustomType. The server will prompt a different product mattering on the CustomType.

RemoteEvents.C2S.Gamepass.CustomId.OnServerEvent:Connect(function(Player,ImageId,CustomType)
	
	local ProductId
	
	if CustomType == "Emote" then
		ProductId = GamepassFolder.CustomIds.CustomEmote.ProductId.Value
		
	elseif CustomType == "BannerIcon" then
		ProductId = ReplicatedStorage.Gamepasses.CustomIds.CustomBannerIcon.ProductId.Valuee
	end

	if ProductId ~= nil then--If ProductId isn't an empty value
		MarketplaceService:PromptProductPurchase(Player,ProductId)
		
	else
		print("Product id was nil")
	end
	
	
	MarketplaceService.ProcessReceipt = ProcessReceipt(ImageId)--Does function for when the player buys any of the products
end)

This is the function that will run when the player clicks the buy button, which is also MarketplaceService.ProcessReceipt = ProcessReceipt(ImageId) from the remote event. Mattering on the product id, a Player's value stored inside them will be change to the ImageId string.

Problem is that it tells me 1 argument is missing or nil, which I don't quite really understand.

function ProcessReceipt(ReceiptInfo, ImageId)
	
	local Player = Players:GetPlayerByUserId(ReceiptInfo.PlayerId)
	
	if Player then
		
			
		if ReceiptInfo.ProductId == ReplicatedStorage.Gamepasses.CustomIds.CustomEmote.ProductId.Value then
			
			Player.Essentials.Equipped.Emotes.CustomEmote.Value = ImageId
			print("Successfully changed Custom Emote")
		elseif ReceiptInfo.ProductId == ReplicatedStorage.Gamepasses.CustomIds.CustomBannerIcon.ProductId.Value then
			
			Player.Essentials.Equipped.Banner.CustomBannerIcon.Value = ImageId
			print("Successfully changed Banner Icon")
		end
		
		return Enum.ProductPurchaseDecision.PurchaseGranted--Allows purchase
	end
	
end

Here is the error

Anonymous