LT version? The error appears when your file contains AEC objects from another AutoCAD variant like Civil or Architecture and you are trying to save the file in a numerci version lower than what was created (for example and LT2010 file being saved down to an LT2007 file format).
Do you need the AEC content to remain intact? Do you even know if you want them at all? Ask before you are directed on how to obliterate them in the next follow up email from someone around here.