pyflakes3: 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/pyflakes3 - from package: pyflakes

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

Error in other languages:
pyflakes3: bevel nie gevind nie
pyflakes3: comandă negăsită
pyflakes3: comando não encontrado
pyflakes3: commande introuvable
pyflakes3: command not found
pyflakes3: käsku ei ole
pyflakes3: không tìm thấy lệnh
pyflakes3: komanda nerasta
pyflakes3: Kommando nicht gefunden.
pyflakes3: kommandot finns inte
pyflakes3: komut yok
pyflakes3: nie znaleziono polecenia
pyflakes3: níor aimsíodh an t-ordú
pyflakes3: no se encontró la orden
pyflakes3: no s'ha trobat l'ordre
pyflakes3: opdracht niet gevonden
pyflakes3: parancs nem található
pyflakes3: perintah tidak ditemukan
pyflakes3: príkaz nenájdený
pyflakes3: příkaz nenalezen
pyflakes3: команда не найдена
pyflakes3: командата не е открита
pyflakes3: 命令找不到
pyflakes3: コマンドが見つかりません