The "Total Balance" you see on your wallet page shows the entirety of the assets in your account. However, in order to perform a new transaction, this amount must be in the "Available Balance" section. If you have a balance but cannot sell, the main reason is as follows:
Open Order Block: If there is a buy/sell request (open order) in your account that has not yet been executed, the system places a temporary block on the asset in the amount of that order and deducts the amount from your available balance.
To remove the block on your balance and use your assets freely again, simply cancel your open order:
You can view and cancel your pending transactions by following the steps "Buy/Sell > My Open Orders" or "Orders > Open Orders." The block will be lifted instantly, and your balance will become available.