manage.py报错 ModuleNotFoundError No module named 'django'

出现场景：我在Windows系统写好了一个django项目，复制到Ubuntu中,运行python3 manage.py migrate一切正常，但运行python3 manage.py runserver报错：

Traceback (most recent call last):
File “manage.py”, line 11, 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 22, in
main()
File “manage.py”, line 13, in main
raise ImportError(
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?

在终端中进入python3，import django也不报错。网上很多方法，要么需要安装虚拟环境太复杂，要么完全无效。我突然想到可能是Linux环境下创建项目时的manage.py有不同，于是创建项目django-admin startproject my_project，提示：

安装完成以后，神奇地发现，python3 manage.py runserver成功运行了！