validate: 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/sbin/validate - from package: mod_auth_shadow

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

On Debian possible default full paths are:

/usr/bin/Validate - from package: haxml
/usr/bin/Validate - from package: wdg-html-validator

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

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