What is happening
You try to close a trade and it will not go through — an error appears, or nothing happens. Most causes are the same as those that block opening an order, plus a few specific to closing. Work through them methodically.
Common causes
No connection, so the close command cannot reach the server. The market is closed for that symbol, so no price is available to trade against. An 'off quotes' or requote as price moves in a fast market. Another command already processing on the order ('trade context busy' or 'order is locked'). Trading disabled on the account. Or a platform freeze.
Step-by-step fix
Check the connection status first. Confirm the symbol's market is open. Increase the allowed deviation and try again if price is moving. Wait a moment if another command is processing, and avoid double-clicking. If the platform is frozen, restart it and re-check whether the trade is still open before acting.
If nothing works and you must limit risk, some brokers let you close via a web or mobile version of the platform — try that as a backup route.
How to prevent it
Keep a stable connection, know your symbols' hours, and have a backup way to access your account (mobile or web) in case the desktop platform fails at a bad moment.
Broker's fault or yours?
Connection, closed markets, and moving prices are usually not the broker's fault. But being unable to close during open hours on a good connection, with trading mysteriously disabled, is serious — document it and escalate, because being trapped in a position is one of the strongest reasons to distrust a broker.