heroku config:set DISABLE_COLLECTSTATIC=1 を実行してもエラーが消えません。

前提・実現したいこと

Djangoを用いて作成したWEBアプリをHerokuへデプロイしたのですが、

ブラウザにApplication error画面が表示されました。

エラーの詳細を確認しようと「heroku logs --tail」を実行すると、以下の内容が表示されました。

発生している問題・エラーメッセージ

  Build LogID d41756b3-3ebc-40e7-a47f-ea9767cbd718
-----> Python app detected
 !     Python has released a security update! Please consider upgrading to python-3.6.10
       Learn More: https://devcenter.heroku.com/articles/python-runtimes
-----> Installing python-3.6.6
-----> Installing pip
-----> Installing SQLite3
-----> Installing requirements with pip
       Collecting asgiref==3.2.7
         Downloading asgiref-3.2.7-py2.py3-none-any.whl (19 kB)
       Collecting colorama==0.4.3
         Downloading colorama-0.4.3-py2.py3-none-any.whl (15 kB)
       Collecting dj-database-url==0.5.0
         Downloading dj_database_url-0.5.0-py2.py3-none-any.whl (5.5 kB)
       Collecting Django==3.0.6
         Downloading Django-3.0.6-py3-none-any.whl (7.5 MB)
       Collecting django-heroku==0.3.1
         Downloading django_heroku-0.3.1-py2.py3-none-any.whl (6.2 kB)
       Collecting django-widget-tweaks==1.4.8
         Downloading django_widget_tweaks-1.4.8-py2.py3-none-any.whl (11 kB)
       Collecting gunicorn==20.0.4
         Downloading gunicorn-20.0.4-py2.py3-none-any.whl (77 kB)
       Collecting Pillow==7.1.2
         Downloading Pillow-7.1.2-cp36-cp36m-manylinux1_x86_64.whl (2.1 MB)
       Collecting psycopg2==2.8.5
         Downloading psycopg2-2.8.5.tar.gz (380 kB)
       Collecting pytz==2020.1
         Downloading pytz-2020.1-py2.py3-none-any.whl (510 kB)
       Collecting qrcode==6.1
         Downloading qrcode-6.1-py2.py3-none-any.whl (31 kB)
       Collecting six==1.15.0
         Downloading six-1.15.0-py2.py3-none-any.whl (10 kB)
       Collecting sqlparse==0.3.1
         Downloading sqlparse-0.3.1-py2.py3-none-any.whl (40 kB)
       Collecting whitenoise==5.1.0
         Downloading whitenoise-5.1.0-py2.py3-none-any.whl (19 kB)
       Collecting XlsxWriter==1.2.8
         Downloading XlsxWriter-1.2.8-py2.py3-none-any.whl (141 kB)
       Building wheels for collected packages: psycopg2
         Building wheel for psycopg2 (setup.py): started
         Building wheel for psycopg2 (setup.py): finished with status 'done'
         Created wheel for psycopg2: filename=psycopg2-2.8.5-cp36-cp36m-linux_x86_64.whl size=447325 sha256=d253d9f74aac2ac002345746b3379752db16c271487c6e0c5b02a81ee54f045a
         Stored in directory: /tmp/pip-ephem-wheel-cache-23d0d72r/wheels/52/9a/ab/22a0d1dc560ad5c0d09cc1c968015f89726e7e01c190dee110
       Successfully built psycopg2
       Installing collected packages: asgiref, colorama, dj-database-url, sqlparse, pytz, Django, psycopg2, whitenoise, django-heroku, django-widget-tweaks, gunicorn, Pillow, six, qrcode, XlsxWriter
       Successfully installed Django-3.0.6 Pillow-7.1.2 XlsxWriter-1.2.8 asgiref-3.2.7 colorama-0.4.3 dj-database-url-0.5.0 django-heroku-0.3.1 django-widget-tweaks-1.4.8 gunicorn-20.0.4 psycopg2-2.8.5 pytz-2020.1 qrcode-6.1 six-1.15.0 sqlparse-0.3.1 whitenoise-5.1.0
-----> $ python manage.py collectstatic --noinput
       Traceback (most recent call last):
         File "manage.py", line 21, in <module>
           main()
         File "manage.py", line 17, in main
           execute_from_command_line(sys.argv)
         File "/app/.heroku/python/lib/python3.6/site-packages/django/core/management/__init__.py", line 401, in execute_from_command_line
           utility.execute()
         File "/app/.heroku/python/lib/python3.6/site-packages/django/core/management/__init__.py", line 345, in execute
           settings.INSTALLED_APPS
         File "/app/.heroku/python/lib/python3.6/site-packages/django/conf/__init__.py", line 76, in __getattr__
           self._setup(name)
         File "/app/.heroku/python/lib/python3.6/site-packages/django/conf/__init__.py", line 63, in _setup
           self._wrapped = Settings(settings_module)
         File "/app/.heroku/python/lib/python3.6/site-packages/django/conf/__init__.py", line 142, in __init__
           mod = importlib.import_module(self.SETTINGS_MODULE)
         File "/app/.heroku/python/lib/python3.6/importlib/__init__.py", line 126, in import_module
           return _bootstrap._gcd_import(name[level:], package, level)
         File "<frozen importlib._bootstrap>", line 994, in _gcd_import
         File "<frozen importlib._bootstrap>", line 971, in _find_and_load
         File "<frozen importlib._bootstrap>", line 955, in _find_and_load_unlocked
         File "<frozen importlib._bootstrap>", line 665, in _load_unlocked
         File "<frozen importlib._bootstrap_external>", line 678, in exec_module
         File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
         File "/tmp/build_26f72b00654616a48db1954e5de56524/bisru/settings.py", line 153, in <module>
           SECRET_KEY = os.environ.get['SECRET_KEY']
       TypeError: 'method' object is not subscriptable
 !     Error while running '$ python manage.py collectstatic --noinput'.
       See traceback above for details.
       You may need to update application code to resolve this error.
       Or, you can disable collectstatic for this application:
          $ heroku config:set DISABLE_COLLECTSTATIC=1
       https://devcenter.heroku.com/articles/django-assets
 !     Push rejected, failed to compile Python app.
 !     Push failed

該当のソースコード

python
1# settings.py
2STATIC_URL = '/static/'
3STATICFILES_DIRS = (
4    os.path.join(BASE_DIR, "static"),
5)
6STATIC_ROOT = os.path.join(BASE_DIR, 'staticfiles')