This means that your browser is lying to you and the unit you are trying to give orders to has been moved or been destroyed, but your browser is out of synch.
You may be able to clear your browser cache, close, and restart your browser, and get the server to send you updated data.
If that doesn't work please send a bug report from the menu in the lower right corner of your game and a game operator can reset the server and your connection to it that will resolve the issue. Due to a high volume of tickets this usually takes some time as tickets are worked in the order they are received.