pi_stress: 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/pi_stress - from package: rt-tests

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

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