xmllint: 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/xmllint - from package: libxml2

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

On Debian possible default full paths are:

/usr/bin/xmllint - from package: libxml2-utils

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

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