Are you using the serial number corresponding to the 2010 version, or a serial number corresponding to an earlier version of your license? Please note that, starting with the 2009 version, a new serial number is generated for each version. The system would therefore not allow you to activate a 2010 version with a 2009 serial number.
Do you know if this AutoCAD LT license has been included in a Subscription contract at some point? If the 2010 version was automatically upgraded to a more recent version (as a benefit of the Subscription program) but the Subscription is now expired, then you would indeed receive this kind of error.
The best way for you to know if and how you can activate your AutoCAD LT 2010 is to contact our Registration & Activation team through this online form. Please remember to provide them with your serial number.