please how i can solve this error?

[tyler1677]

ubuntu@sdnhubvm:/ryu-master/ryu/app/openstate/ddos[00:07]$ ryu-manager ddos.pyTraceback (most recent call last):
File "/usr/local/bin/ryu-manager", line 5, in
from pkg_resources import load_entry_point
File "/usr/lib/python2.7/dist-packages/pkg_resources.py", line 2749, in
working_set = WorkingSet._build_master()
File "/usr/lib/python2.7/dist-packages/pkg_resources.py", line 446, in _build_master
return cls._build_from_requirements(requires)
File "/usr/lib/python2.7/dist-packages/pkg_resources.py", line 459, in _build_from_requirements
dists = ws.resolve(reqs, Environment())
File "/usr/lib/python2.7/dist-packages/pkg_resources.py", line 628, in resolve
raise DistributionNotFound(req)
pkg_resources.DistributionNotFound: eventlet>=0.15
ubuntu@sdnhubvm:/ryu-master/ryu/app/openstate/ddos[00:07]$