rackdiag3: 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/rackdiag3 - from package: python3-nwdiag

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

Error in other languages:
rackdiag3: bevel nie gevind nie
rackdiag3: comandă negăsită
rackdiag3: comando não encontrado
rackdiag3: commande introuvable
rackdiag3: command not found
rackdiag3: käsku ei ole
rackdiag3: không tìm thấy lệnh
rackdiag3: komanda nerasta
rackdiag3: Kommando nicht gefunden.
rackdiag3: kommandot finns inte
rackdiag3: komut yok
rackdiag3: nie znaleziono polecenia
rackdiag3: níor aimsíodh an t-ordú
rackdiag3: no se encontró la orden
rackdiag3: no s'ha trobat l'ordre
rackdiag3: opdracht niet gevonden
rackdiag3: parancs nem található
rackdiag3: perintah tidak ditemukan
rackdiag3: príkaz nenájdený
rackdiag3: příkaz nenalezen
rackdiag3: команда не найдена
rackdiag3: командата не е открита
rackdiag3: 命令找不到
rackdiag3: コマンドが見つかりません