mkhtmlindex: 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/mkhtmlindex - from package: imake

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

On Debian possible default full paths are:

/usr/bin/mkhtmlindex - from package: xutils-dev

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

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