zope-testrunner3: 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/zope-testrunner3 - from package: python3-zope.testrunner

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

Error in other languages:
zope-testrunner3: bevel nie gevind nie
zope-testrunner3: comandă negăsită
zope-testrunner3: comando não encontrado
zope-testrunner3: commande introuvable
zope-testrunner3: command not found
zope-testrunner3: käsku ei ole
zope-testrunner3: không tìm thấy lệnh
zope-testrunner3: komanda nerasta
zope-testrunner3: Kommando nicht gefunden.
zope-testrunner3: kommandot finns inte
zope-testrunner3: komut yok
zope-testrunner3: nie znaleziono polecenia
zope-testrunner3: níor aimsíodh an t-ordú
zope-testrunner3: no se encontró la orden
zope-testrunner3: no s'ha trobat l'ordre
zope-testrunner3: opdracht niet gevonden
zope-testrunner3: parancs nem található
zope-testrunner3: perintah tidak ditemukan
zope-testrunner3: príkaz nenájdený
zope-testrunner3: příkaz nenalezen
zope-testrunner3: команда не найдена
zope-testrunner3: командата не е открита
zope-testrunner3: 命令找不到
zope-testrunner3: コマンドが見つかりません