Google Earth does not have the kind of analytical tools to create a precise buffer for you. If you need it to be exact, then you'll probably want to try some GIS software like QGIS.
That said, if you only need it to be roughly accurate, and your line isn't too complex, then you could use a combination of the measuring and drawing tools in Google Earth Pro. Start with your path/line, and use the circle measure tool to draw circles of your desired radius with their centers at each end of the line. Save each circle to your map. Then draw and save similar circles centered on each vertex of your line. Finally, use the shape/polygon drawing tool to sketch your buffer, using the circles as a guide.