Getting "ImportError: without a module named wnf" during ERP-Next installation

I try to install erpnext by running 'install_erpnext.py' and it gives an error message:

root@erpnext:~# python2.7 install_erpnext.py --create_user
useradd: user 'erpnext' already exists
Enter new UNIX password: Retype new UNIX password: passwd: password updated succ                                                 essfully
Operating System = Linux
Python Version = 2.7.3
Distribution =  ubuntu
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Installing Packages: (This may take some time)
python python-setuptools python-dev build-essential python-mysqldb git memcached                                                 ntp vim screen htop
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MySQL Root user Password:
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Installing Python Modules: (This may take some time)
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Pre-requisites Installed
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Installing ERPNext
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ERPNext Database Name: erpnext
Traceback (most recent call last):
  File "install_erpnext.py", line 301, in <module>
  File "install_erpnext.py", line 39, in install
  File "install_erpnext.py", line 177, in install_erpnext
ImportError: No module named wnf

I do not know what to do now. I am also trying to copy the file "erpnext / lib / wnf.py" and put it using "install_erpnext.py", but it still gave the same error. I have already installed Python modules and other packages. thanks in advance...