I need to use the Buffer in the Editor menu of ArcMap, but it is unavailable.
What is the problem?
You must have to select your polygon(s) to make 'buffer' option enable in Editor Toolbar. You may use 'Select Features' tool or alternatively select your polygon(s) via 'Edit tool' to enable buffer option.
You may also use attribute table for polygon feature selection or use 'Buffer' tool available in ArcGIS to buffer around input polygons.