html2: 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/html2 - from package: xml2

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

On Debian possible default full paths are:

/usr/bin/html2 - from package: xml2

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

Error in other languages:
html2: bevel nie gevind nie
html2: comandă negăsită
html2: comando não encontrado
html2: commande introuvable
html2: command not found
html2: käsku ei ole
html2: không tìm thấy lệnh
html2: komanda nerasta
html2: Kommando nicht gefunden.
html2: kommandot finns inte
html2: komut yok
html2: nie znaleziono polecenia
html2: níor aimsíodh an t-ordú
html2: no se encontró la orden
html2: no s'ha trobat l'ordre
html2: opdracht niet gevonden
html2: parancs nem található
html2: perintah tidak ditemukan
html2: príkaz nenájdený
html2: příkaz nenalezen
html2: команда не найдена
html2: командата не е открита
html2: 命令找不到
html2: コマンドが見つかりません