An incompatible version of DWG file

One of the most frequently asked questions when using a Block library with DWG files is how to open a newer-version DWG file format in my old version of AutoCAD?

The AutoCAD file format - DWG - is updated in every 3rd release of AutoCAD. So published DWG blocks saved in a newer version might not be readable in an old AutoCAD version. E.g. AutoCAD 2006 cannot open drawings from AutoCAD 2009.

But the solution is simple, you don't need to upgrade your AutoCAD immediately - just install the free tool - Autodesk DWG TrueView (2008 or higher), don't get confused by its name (it can really convert, not only view) and use its conversion functionality to save back the complaining DWG block to any older DWG format. There is no other option for this - various "DWG conversion" utilities do not save the genuine DWG format and the resulting DWG files can cause many problems in AutoCAD.

See the tips:
How to convert an AutoCAD DWG drawing to an older file format?
Error message: File was created by an incompatible version of AutoCAD