Xnest: 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/Xnest - from package: xorg-x11-server-Xnest

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

On Debian possible default full paths are:

/usr/bin/Xnest - from package: xnest

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

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