nwdiag: 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/nwdiag - from package: python-nwdiag

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

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