How to use PromptPurchaseFinished?

I need to print when product is purchased; the problem is that the function does not even print.

MarketService.MarketService.PromptPurchaseFinished:connect(function(c, a, isPurchased)
	print(c,a,isPurchased)
	if isPurchased then
		table.insert(server.que,server.eventInst:InvokeClient(c,'get_id'))
		
		server.eventInst:InvokeClient(c,'clear_id')
		server.eventInst:InvokeClient(c,'change_desc','- Successful -')
	else
		server.eventInst:InvokeClient(c,'change_desc','- ERROR -')
	end
end)(function(c, a, isPurchased)
	print(c,a,isPurchased)
	if isPurchased then
		-- DO STUFF HERE
	else
		-- DO STUFF HERE
	end
end)
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