stress-ng: 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/stress-ng - from package: stress-ng

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

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