pytest3: 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/pytest3 - from package: python3-logilab-common

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

Error in other languages:
pytest3: bevel nie gevind nie
pytest3: comandă negăsită
pytest3: comando não encontrado
pytest3: commande introuvable
pytest3: command not found
pytest3: käsku ei ole
pytest3: không tìm thấy lệnh
pytest3: komanda nerasta
pytest3: Kommando nicht gefunden.
pytest3: kommandot finns inte
pytest3: komut yok
pytest3: nie znaleziono polecenia
pytest3: níor aimsíodh an t-ordú
pytest3: no se encontró la orden
pytest3: no s'ha trobat l'ordre
pytest3: opdracht niet gevonden
pytest3: parancs nem található
pytest3: perintah tidak ditemukan
pytest3: príkaz nenájdený
pytest3: příkaz nenalezen
pytest3: команда не найдена
pytest3: командата не е открита
pytest3: 命令找不到
pytest3: コマンドが見つかりません