You need to break it down to points if they're good points and reconstruct. Polylines are made from paths, polygons are made from rings. Although they are created in a similar way they are not compatable, hence your error.

Go through each point on the line adding a point to your output array and then insert.

here's a post that might help http://gis.stackexchange.com/questions/26369/get-all-the-points-of-a-polyline

This should work, though it's only a fragment... you have to set your own FeatureClass and make an insert cursor to accept the polygon:

    rows = arcpy.da.SearchCursor(FeatureClass)
    desc = arcpy.Describe(FeatureClass)
    shapefieldname = desc.ShapeFieldName
    
    for row in rows:
    	feat = row.getValue(shapefieldname) # input line
    	PArray = arcpy.Array() # new polygon
    	partnum = 0
    	for part in feat:
    		for pnt in feat.getPart(partnum):
    			PArray.add(arcpy.Point(pnt.X,pnt.Y))
    		# you will need to check that the first and last point are the same
    		partnum += 1
    	OutPoly = arcpy.Polygon(PArray) # now it's a polygon

Have a read of these:

Reading Geometries http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#//002z0000001t000000

Writing Geometries http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#/Writing_geometries/002z0000001v000000/