gnunet-nat-server: 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/gnunet-nat-server - from package: gnunet

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

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