Why is there an error when launching gunicorn?

Question

Django

when launching gunicorn, this is the error:

Traceback (most recent call last):
  File "/usr/bin/gunicorn", line 5, in <module>
    from pkg_resources import load_entry_point
  File "build/bdist.linux-x86_64/egg/pkg_resources/__init__.py", line 3019, in <module>
  File "build/bdist.linux-x86_64/egg/pkg_resources/__init__.py", line 3003, in _call_aside
  File "build/bdist.linux-x86_64/egg/pkg_resources/__init__.py", line 3032, in _initialize_master_working_set
  File "build/bdist.linux-x86_64/egg/pkg_resources/__init__.py", line 655, in _build_master
  File "build/bdist.linux-x86_64/egg/pkg_resources/__init__.py", line 963, in require
  File "build/bdist.linux-x86_64/egg/pkg_resources/__init__.py", line 849, in resolve
pkg_resources.DistributionNotFound: The 'gunicorn==19.4.5' distribution was not found and is required by the application