posh: 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:

/bin/posh - from package: posh

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

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