app.py: 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/lib/python3/dist-packages/celery/tests/bin/proj/app.py - from package: python3-celery
/usr/lib/python2.7/dist-packages/celery/tests/bin/proj/app.py - from package: python-celery

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

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