nwdiag3: 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/nwdiag3 - from package: python3-nwdiag

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

Error in other languages:
nwdiag3: bevel nie gevind nie
nwdiag3: comandă negăsită
nwdiag3: comando não encontrado
nwdiag3: commande introuvable
nwdiag3: command not found
nwdiag3: käsku ei ole
nwdiag3: không tìm thấy lệnh
nwdiag3: komanda nerasta
nwdiag3: Kommando nicht gefunden.
nwdiag3: kommandot finns inte
nwdiag3: komut yok
nwdiag3: nie znaleziono polecenia
nwdiag3: níor aimsíodh an t-ordú
nwdiag3: no se encontró la orden
nwdiag3: no s'ha trobat l'ordre
nwdiag3: opdracht niet gevonden
nwdiag3: parancs nem található
nwdiag3: perintah tidak ditemukan
nwdiag3: príkaz nenájdený
nwdiag3: příkaz nenalezen
nwdiag3: команда не найдена
nwdiag3: командата не е открита
nwdiag3: 命令找不到
nwdiag3: コマンドが見つかりません