xlsclients: 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/xlsclients - from package: xorg-x11-utils

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

On Debian possible default full paths are:

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

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

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