mkindex: 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 CentOS possible default full paths are:

/usr/bin/mkindex - from package: texlive

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/mkindex - from package: texlive-binaries
/usr/lib/wims/bin/mkindex - from package: wims

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

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