openbox-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/openbox-session - from package: openbox

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

On Debian possible default full paths are:

/usr/bin/openbox-session - from package: openbox

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

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