Standard AutoCAD cannot read KML. Use a free online converter or GIS software.
KML files often include "Altitude" data. If your lines look strange, check them in a 3D view; you may need to "Flatten" the objects if you only need a 2D site plan. how to import kml to autocad
If the data is complex, it is often best to convert the KML to a DXF (Drawing Exchange Format) using GIS software first. Standard AutoCAD cannot read KML