gnunet-nse: 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-nse - from package: gnunet

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

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