Either you've used the wrong name or code. (Check the spelling. Capital letters need to be wrote in capital letters.) Or the ship has been moved already, and the code is invalid.
Or there's no space on your account (5 ships is the maximum).
if you are on the account when a ship gets transferred out you get kicked for it to go through (like with a rename), but I haven't heard that it causes the move to fail...at least that what happened to me last time and the purchaser hasn't complained something went wrong.