用老师的方法，新建不了应用！怎么办？？？

(base) JACKWONGdeMBP:django_introduction jackwong$ python manage.py startapp blog

Traceback (most recent call last):

  File "manage.py", line 10, in main

    from django.core.management import execute_from_command_line

ModuleNotFoundError: No module named 'django'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):

  File "manage.py", line 21, in <module>

    main()

  File "manage.py", line 16, in main

    ) from exc

ImportError: Couldn't import Django. Are you sure it's installed and available on your PYTHONPATH environment variable? Did you forget to activate a virtual environment?

(base) JACKWONGdeMBP:django_introduction jackwong$