ck-list-sessions: 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/ck-list-sessions - from package: ConsoleKit

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

On Debian possible default full paths are:

/usr/bin/ck-list-sessions - from package: ConsoleKit

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

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