mkgrkindex: command not found

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

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

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/mkgrkindex - from package: texlive-lang-greek

You may install the required package with command apt-get install {package}

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