Check your account balance on the exchange i.e. Binance. Does the ETH/BTC etc. match the ETH/BTC etc. of the trade you want to sell? Most of the time, "unknown error when trying to sell transaction" means that you don't have enough cryptocurrency on the exchange to execute the trade. This can be caused by doing manual actions, (selling / buying), on the exchange and depleting the amount needed to close the transfer.
You can go into your exchange and either manually sell the position or restore the proper balances.