django.db.utils.OperationalError: unable to open database file error

I am new to Django. I've installed in virtual env in ubuntu. Whenever i am trying to run

python3 manage.py runserver

i am getting the following error

File "/home/aashish/Python/Django/project_env/lib/python3.6/site-packages/django/db/backends/sqlite3/base.py", line 199, in get_new_connection
    conn = Database.connect(**conn_params)
django.db.utils.OperationalError: unable to open database file

This is my database code in settings.py

DATABASES = {
    'default': {
        'ENGINE': 'django.db.backends.sqlite3',
        'NAME': os.path.join(BASE_DIR, 'db.sqlite3'),
    }
}

How to debug this? Thank you in advance!