I get this error

root@server1 [/home/toaditya/public_html/erp/openerp]# **openerp-server**
Traceback (most recent call last):
  File "/usr/local/bin/openerp-server", line 4, in <module>
    import pkg_resources
  File "/usr/local/lib/python2.7/site-packages/pkg_resources.py", line 2793, in <module>
    working_set.require(__requires__)
  File "/usr/local/lib/python2.7/site-packages/pkg_resources.py", line 673, in require
    needed = self.resolve(parse_requirements(requirements))
  File "/usr/local/lib/python2.7/site-packages/pkg_resources.py", line 576, in resolve
    raise DistributionNotFound(req)
**pkg_resources.DistributionNotFound: lxml**

what could be the issue