pyflakes: 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 CentOS possible default full paths are:

/usr/bin/pyflakes - from package: pyflakes

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/pyflakes - from package: pyflakes

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

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