Heroku Dash Application deploy error : gunicorn.errors.HaltServer

Attempting to deploy a dash application on heroku and running into an error. My index.py has server = app.server variable defined.

And Procfile is: web: gunicorn index:app.server

Here’s is the full traceback:

File "/app/.heroku/python/lib/python3.6/site-packages/gunicorn/arbiter.py", line 242, in handle_chld
2021-02-11T13:31:13.249430+00:00 app[web.1]: self.reap_workers()
2021-02-11T13:31:13.249435+00:00 app[web.1]: File "/app/.heroku/python/lib/python3.6/site-packages/gunicorn/arbiter.py", line 528, in reap_workers
2021-02-11T13:31:13.249715+00:00 app[web.1]: raise HaltServer(reason, self.APP_LOAD_ERROR)
2021-02-11T13:31:13.249755+00:00 app[web.1]: gunicorn.errors.HaltServer: <HaltServer 'App failed to load.' 4>
2021-02-11T13:31:13.249759+00:00 app[web.1]: 
2021-02-11T13:31:13.249760+00:00 app[web.1]: During handling of the above exception, another exception occurred:
2021-02-11T13:31:13.249760+00:00 app[web.1]: 
2021-02-11T13:31:13.249784+00:00 app[web.1]: Traceback (most recent call last):
2021-02-11T13:31:13.249787+00:00 app[web.1]: File "/app/.heroku/python/bin/gunicorn", line 8, in <module>
2021-02-11T13:31:13.249912+00:00 app[web.1]: sys.exit(run())
2021-02-11T13:31:13.249955+00:00 app[web.1]: File "/app/.heroku/python/lib/python3.6/site-packages/gunicorn/app/wsgiapp.py", line 58, in run
2021-02-11T13:31:13.250091+00:00 app[web.1]: WSGIApplication("%(prog)s [OPTIONS] [APP_MODULE]").run()
2021-02-11T13:31:13.250096+00:00 app[web.1]: File "/app/.heroku/python/lib/python3.6/site-packages/gunicorn/app/base.py", line 228, in run
2021-02-11T13:31:13.250285+00:00 app[web.1]: super().run()
2021-02-11T13:31:13.250286+00:00 app[web.1]: File "/app/.heroku/python/lib/python3.6/site-packages/gunicorn/app/base.py", line 72, in run
2021-02-11T13:31:13.250420+00:00 app[web.1]: Arbiter(self).run()
2021-02-11T13:31:13.250425+00:00 app[web.1]: File "/app/.heroku/python/lib/python3.6/site-packages/gunicorn/arbiter.py", line 229, in run
2021-02-11T13:31:13.250603+00:00 app[web.1]: self.halt(reason=inst.reason, exit_status=inst.exit_status)
2021-02-11T13:31:13.250608+00:00 app[web.1]: File "/app/.heroku/python/lib/python3.6/site-packages/gunicorn/arbiter.py", line 342, in halt
2021-02-11T13:31:13.250827+00:00 app[web.1]: self.stop()
2021-02-11T13:31:13.250828+00:00 app[web.1]: File "/app/.heroku/python/lib/python3.6/site-packages/gunicorn/arbiter.py", line 393, in stop
2021-02-11T13:31:13.251055+00:00 app[web.1]: time.sleep(0.1)
2021-02-11T13:31:13.251060+00:00 app[web.1]: File "/app/.heroku/python/lib/python3.6/site-packages/gunicorn/arbiter.py", line 242, in handle_chld
2021-02-11T13:31:13.251249+00:00 app[web.1]: self.reap_workers()
2021-02-11T13:31:13.251254+00:00 app[web.1]: File "/app/.heroku/python/lib/python3.6/site-packages/gunicorn/arbiter.py", line 528, in reap_workers
2021-02-11T13:31:13.251520+00:00 app[web.1]: raise HaltServer(reason, self.APP_LOAD_ERROR)
2021-02-11T13:31:13.251553+00:00 app[web.1]: gunicorn.errors.HaltServer: <HaltServer 'App failed to load.' 4>