我是writing some code for a project that requires virtualenvwrapper，我不能让我的开发工作正常运行，我相信由于Fabric的问题。我在OS X Yosemeti上使用Python 2.7.2（32位），我相信已经正确地遵循了项目的开发人员文档。
在我的virtualenv环境中，我满足了所有要求：
Requirement already satisfied (use --upgrade to upgrade): Flask==0.10.1 in /Users/benjamin/.virtualenvs/juxtapose/lib/python2.7/site-packages (from -r requirements.txt (line 1))
Requirement already satisfied (use --upgrade to upgrade): Jinja2==2.7 in /Users/benjamin/.virtualenvs/juxtapose/lib/python2.7/site-packages (from -r requirements.txt (line 2))
Requirement already satisfied (use --upgrade to upgrade): MarkupSafe==0.18 in /Users/benjamin/.virtualenvs/juxtapose/lib/python2.7/site-packages (from -r requirements.txt (line 3))
Requirement already satisfied (use --upgrade to upgrade): Werkzeug==0.9.3 in /Users/benjamin/.virtualenvs/juxtapose/lib/python2.7/site-packages (from -r requirements.txt (line 4))
Requirement already satisfied (use --upgrade to upgrade): boto==2.27.0 in /Users/benjamin/.virtualenvs/juxtapose/lib/python2.7/site-packages (from -r requirements.txt (line 5))
Requirement already satisfied (use --upgrade to upgrade): itsdangerous==0.22 in /Users/benjamin/.virtualenvs/juxtapose/lib/python2.7/site-packages (from -r requirements.txt (line 6))
Requirement already satisfied (use --upgrade to upgrade): paramiko==1.10.1 in /Users/benjamin/.virtualenvs/juxtapose/lib/python2.7/site-packages (from -r requirements.txt (line 7))
Requirement already satisfied (use --upgrade to upgrade): ply==3.4 in /Users/benjamin/.virtualenvs/juxtapose/lib/python2.7/site-packages (from -r requirements.txt (line 8))
Requirement already satisfied (use --upgrade to upgrade): pycrypto==2.6 in /Users/benjamin/.virtualenvs/juxtapose/lib/python2.7/site-packages (from -r requirements.txt (line 9))
Requirement already satisfied (use --upgrade to upgrade): wsgiref==0.1.2 in /Library/Frameworks/Python.framework/Versions/7.1/lib/python2.7 (from -r requirements.txt (line 10))
Requirement already satisfied (use --upgrade to upgrade): gunicorn==19.3.0 in /Users/benjamin/.virtualenvs/juxtapose/lib/python2.7/site-packages (from -r requirements.txt (line 11))
Requirement already satisfied (use --upgrade to upgrade): Fabric==1.6.1 in /Users/benjamin/.virtualenvs/juxtapose/lib/python2.7/site-packages/Fabric-1.6.1-py2.7.egg (from -r requirements.txt (line 12))
Cleaning up...
然而，当我运行fab serve时，我收到了错误
(juxtapose)benjamin:juxtapose benjamin$ fab serve
Warning: Command(s) not found:
serve
我原本希望被发送到一个网站，例如localhost：4000我跑fab serve时。你知道为什么我会得到这个错误以及我如何解决它？
由于