after run sudo pip install -r /opt/odoo/requirements.txt
this one error 
i reinstall with new ubuntu but not work at all 
 Rolling back uninstall of lxml
Command "/usr/bin/python -u -c "import setuptools, tokenize;__file__='/tmp/pip-build-1bNupZ/lxml/setup.py';f=getattr(tokenize, 'open', open)(__file__);code=f.read().replace('\r\n', '\n');f.close();exec(compile(code, __file__, 'exec'))" install --record /tmp/pip-jN615y-record/install-record.txt --single-version-externally-managed --compile" failed with error code 1 in /tmp/pip-build-1bNupZ/lxml/