DWG support is still very rudimentary in LibreCAD, so chances are big, that libdxfrw, our import/export library, fails to read the DWG file. Sadly the error detection and reporting is not existent too, so that the error message is misleading.
If you have access to AutoCAD, it's recommend to save the drawing in DXF format, which is much better supported yet.
When your DWG drawing doesn't contain any secret parts, you can post it here, so we can take a look where the import fails.
Armin
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