xmonad-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 Debian possible default full paths are:

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

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

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