mono-upnp-gtk: 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/mono-upnp-gtk - from package: mono-upnp-bin

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

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