google.auth.exceptions.DefaultCredentialsError: File was not found

I am running my tests on Circle CI using pytest.

However, it seems that it can not find the key.json file to authenticate my client library to GCP.

../.cache/pypoetry/virtualenvs/nozzle-3aSsmiER-py3.7/lib/python3.7/site-packages/google/auth/_default.py:219: in _get_explicit_environ_credentials
    os.environ[environment_vars.CREDENTIALS], quota_project_id=quota_project_id
../.cache/pypoetry/virtualenvs/nozzle-3aSsmiER-py3.7/lib/python3.7/site-packages/google/auth/_default.py:118: in load_credentials_from_file
    "File {} was not found.".format(filename)
E   google.auth.exceptions.DefaultCredentialsError: File  was not found.
__________ ERROR collecting tests/nozzle_app/users/test_users_app.py

How can I solve this issue ?