First, let’s address the elephant in the room. If you download the classic (the last official stable release) and point it at a Minecraft 1.16.5 world, you will encounter immediate issues:
Locate the area you wish to edit. In 1.16.5, if you are in the Nether, be aware that the bedrock ceiling (Y=128) can obstruct your view. Use the "Slice" tool to cut the view height if necessary. mcedit 1.16.5
MCEdit is officially no longer compatible with Minecraft versions 1.13 and higher, including First, let’s address the elephant in the room