gnome-session-classic: 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 Debian possible default full paths are:

/usr/bin/gnome-session-classic - from package: gnome-shell-extensions

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

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