flake8: 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/flake8 - from package: python-flake8

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

Error in other languages:
flake8: bevel nie gevind nie
flake8: comandă negăsită
flake8: comando não encontrado
flake8: commande introuvable
flake8: command not found
flake8: käsku ei ole
flake8: không tìm thấy lệnh
flake8: komanda nerasta
flake8: Kommando nicht gefunden.
flake8: kommandot finns inte
flake8: komut yok
flake8: nie znaleziono polecenia
flake8: níor aimsíodh an t-ordú
flake8: no se encontró la orden
flake8: no s'ha trobat l'ordre
flake8: opdracht niet gevonden
flake8: parancs nem található
flake8: perintah tidak ditemukan
flake8: príkaz nenájdený
flake8: příkaz nenalezen
flake8: команда не найдена
flake8: командата не е открита
flake8: 命令找不到
flake8: コマンドが見つかりません