What does "LOCKED_TRACK constraint reserved for Locked Track" mean? I am working on the rig of a jet-fighter with no bones, using empties to emulate a hinge for the different parts that flap up etc; for this I use two empties: one to act as a hinge, the other as target. I applied a locked track constraint to the hinge, but when I select the hinge empty and I apply the Locked track constraint I get the following cryptic error in the properties window where the actual constraint was suppose to be:


  LOCKED_TRACK constraint reserved for Locked Track.


I have researched the internet and I could not find any information on the error, just the wiki manual page that confirms that LOCKED TRACK is exactly what I need. 

The little info I found did not address my problem at all, but ways of using the LOCKED TRACK constraint. Can anybody help?
I never used a Locked track constraint but using shape keys could be an alternative for animations without bones.

If you separate P the flaps and parent them to the plane Ctrl-P you could add two shape keys and animate them by keyframing the value: