App works locally but GAE deploy giving no error detail but only "Error Response: [9] ".Application startup error:

Dash Python
1

HI Alls,

the full ‘error message’ is -

latest: digest: sha256:924cb0473eaef8b63813ce3771f75189138b922745909c818103c34fe682fe29 size: 3460
DONE

Updating service [default] (this may take several minutes)…failed.
ERROR: (gcloud.app.deploy) Error Response: [9]
Application startup error:
[2019-06-18 11:53:51 +0000] [1] [INFO] Starting gunicorn 19.9.0
[2019-06-18 11:53:51 +0000] [1] [INFO] Listening at: http://0.0.0.0:8080 (1)
[2019-06-18 11:53:51 +0000] [1] [INFO] Using worker: sync
[2019-06-18 11:53:51 +0000] [9] [INFO] Booting worker with pid: 9
Application object must be callable.
[2019-06-18 11:53:55 +0000] [9] [INFO] Worker exiting (pid: 9)
[2019-06-18 11:53:55 +0000] [1] [INFO] Shutting down: Master
[2019-06-18 11:53:55 +0000] [1] [INFO] Reason: App failed to load.

my app.yaml file is :

runtime: python

env: flex

entrypoint: gunicorn -b :$PORT app:app

threadsafe: false

Any ideas guys , really stuck here as there in no further detail in my GAE account either ??!
Im using python 2.7 btwe.

Maybe you have an idea @chriddyp …?

Thanks a million !
David.