You can cancel a purchase if you haven't activate it.
You should be able to cancel your purchase in the product's page here in MQL5.com, look on the left side where the purchase button originally was.
Check here: https://www.mql5.com/en/users/auto_cad864afcd71f796368aaf00d9f3b4b2/market
or here: https://www.mql5.com/en/users/auto_cad864afcd71f796368aaf00d9f3b4b2/accounting
You have to check your "Purchases" section where you'll find a list of all tools you ever bought.
hover over the top product and a button will appear.
Please enable the necessary setting in your browser, otherwise you will not be able to log in.