nn: 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/nn - from package: nn

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