gnunet-vpn: command not found

Have You tried accessing the binary with the full path?

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/gnunet-vpn - from package: gnunet

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