You should be using 057F1 for AutoCAD 2014 LT.
If you are still having issues with that Product Key, I would suggest using the Autodesk licensing assistance, please contact their Activation and Registration department directly:
To find self-service solutions or reach them via Chat for Activation and Licensing Help:
1. Visit
http://knowledge.autodesk.com/contactus2. Select “Activation & Registration” or “License Management”
3. Select appropriate subtopics from the pull-down menus
4. Select “I Still Need Help” if you need personal assistance
Good luck!