Django can't find my environment variable? VIEW_DB_USER not found. Declare it as envvar or define a

I am developing an app in django and I pushed it on Heroku.

I want to query my database with SQL in my views.py, so I connect to my database with connect method.

When it comes to deploying my app, I obviously want to hide my database credentials.

So I save them in a .env file in root directory and use the config function to call these variables from my views.py.

However, I run into:

VIEW_DB_USER not found. Declare it as envvar or define a default value.

(error at view_db_user = config("VIEW_DB_USER") )

Here is my views.py:

import os 
import django_heroku 
from decouple import config
import dj_database_url

view_db_user = config("VIEW_DB_USER")
view_db_password = config("view_db_password")
view_db_host = config("view_db_host")
view_db_database = config("view_db_database")

mydb = pg2.connect(user=VIEW_DB_USER, password=view_db_password,
                                host=view_db_host, database=view_db_database)

Here is my .env file:

SECRET_KEY = 'credential_1'

VIEW_DB_USER = 'credential_2'
view_db_password = 'credential_3'
view_db_host = 'credential_4'
view_db_database = 'credential_5'

From what I see, the problem is that the function config is somehow not working properly. But why?

Please note that this does not work both in local and on heroku.

Also note that SECRET_KEY contains a password of another database that perfectly works, being this passwork stored in my .env file.