xmlstarlet: 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/xmlstarlet - from package: xmlstarlet

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

On Debian possible default full paths are:

/usr/bin/xmlstarlet - from package: xmlstarlet

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

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