I’v never seen the unkown command thing, but, Main Menu should be what it is listed under for keybindings, if that isn’t in there, then either pull a keybindings file from another install preferably clean, or you can check the keybindings file see if there is a line in it for main menu. Besides that I don’t know.
Using notepad++ in mine it is line 188
{ “id”: “main_menu”, “category”: “DEFAULTMODE”, “bindings”: [ { “input_method”: “keyboard”, “key”: [ “ESC” ] } ]
}