xrestop: 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/xrestop - from package: xrestop

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

On Debian possible default full paths are:

/usr/bin/xrestop - from package: xrestop

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

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