ck-launch-session: 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-launch-session - from package: ConsoleKit

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

On Debian possible default full paths are:

/usr/bin/ck-launch-session - from package: ConsoleKit

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

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