What it means
'Order is locked' means the platform (or an EA) is already acting on that specific order and will not accept a second command on it until the first one finishes. It prevents two operations colliding on the same ticket.
Common causes
Sending a close or modify command twice in quick succession. An EA and your manual click both targeting the same order. A slow connection stretching out how long the first command holds the lock.
Step-by-step fix
Wait a moment and check the Trade tab to see whether the order actually changed — often the first command did succeed and the second was simply blocked.
If an EA manages that order, avoid touching it manually at the same time; pause AutoTrading first if you need to intervene. If the platform seems stuck, restart it and re-check the order's true state before sending anything else.
How to prevent it
Do not double-click close or modify. Decide whether an order is managed by you or by an EA, not both at once. A stable connection shortens the lock window.
Broker's fault or yours?
This is a local, timing-related lock, so it is on your side. Slow server responses can make it more frequent, which is worth noting if it happens constantly on a good connection.