I have a download version of LT13 and am in the process of upgrading it with the current service pack.
I get and error 1706 - no valid source could be found for the product AutoCAD LT 2013 - English.
Its looking for acadlt.msi on an installation CD which I dont have as I bought the download version.
You could downlod the Service pack at below link:
When you met acadlt.msi unable to be found issue, Could you uninstall and reinstall AutoCAD LT 2013 to avoide this issue?