nngoback: 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/nngoback - from package: nn

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

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