xmessage: 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/xmessage - from package: xorg-x11-apps

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

On Debian possible default full paths are:

/usr/bin/xmessage - from package: x11-utils

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

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