xmltidy: command not found

Have You tried accessing the binary with the full path?

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/xmltidy - from package: perl-XML-Tidy

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

On Debian possible default full paths are:

/usr/bin/xmltidy - from package: libxml-tidy-perl

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