First, try refreshing your page and repeating the action once. Do not press the button multiple times. If this does not work proceed further with this article.
Take a moment to check your cryptocurrency account balance on the exchange i.e. Binance. Does the Cryptocurrency balance of the coin you are trying to sell match or exceed the cryptocurrency amount of the trade you want to sell? In most cases the, "unknown error when trying to sell transaction" means that you don't have enough cryptocurrency on the exchange to execute the trade. This happens if you manually closed/sold the position or cryptocurrency in the exchange or removed it.
This can also be caused by doing manual actions, i.e. (selling/buying), on the exchange and depleting the amount needed to close the transaction. If this happens at any point it can glitch a Trader causing it to not sell the position in the future properly. Always make sure you have over the balance of the coin at all times if you plan to also manually trade inside of your exchange.
Pressing the manual sell command in rapid execution will also cause a Trader to malfunction. Every time you press the manual sell option press it once and allow a few minutes for it to execute the entire process. Keep in mind it is literally sending a sell order to the exchange that must be fulfilled. If your trader is not manually selling you will need to create a new Trader to re-enable this option as it will not fix itself.
Always keep in mind you can always go into your exchange and manually sell the position.