gnunet-conversation: 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-conversation - from package: gnunet

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

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