mighty-mkindex: command not found

Have You tried accessing the binary with the full path?

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On Debian possible default full paths are:

/usr/bin/mighty-mkindex - from package: mighttpd2

You may install the required package with command apt-get install {package}
Error in other languages:
mighty-mkindex: bevel nie gevind nie
mighty-mkindex: comandă negăsită
mighty-mkindex: comando não encontrado
mighty-mkindex: commande introuvable
mighty-mkindex: command not found
mighty-mkindex: käsku ei ole
mighty-mkindex: không tìm thấy lệnh
mighty-mkindex: komanda nerasta
mighty-mkindex: Kommando nicht gefunden.
mighty-mkindex: kommandot finns inte
mighty-mkindex: komut yok
mighty-mkindex: nie znaleziono polecenia
mighty-mkindex: níor aimsíodh an t-ordú
mighty-mkindex: no se encontró la orden
mighty-mkindex: no s'ha trobat l'ordre
mighty-mkindex: opdracht niet gevonden
mighty-mkindex: parancs nem található
mighty-mkindex: perintah tidak ditemukan
mighty-mkindex: príkaz nenájdený
mighty-mkindex: příkaz nenalezen
mighty-mkindex: команда не найдена
mighty-mkindex: командата не е открита
mighty-mkindex: 命令找不到
mighty-mkindex: コマンドが見つかりません