No module named back_ for sqlite backend

Dear Roundup users,

I just try to use the sqlite backend, and when I finish configuration and  start roundup-admin initialise, It complains that 

No module named back_


The Sqlite3 is installed. I attached the log below.

Thanks for you support,

BR,Austin

[austin@support roundup]$ roundup-admin install
Enter tracker home: /opt/roundup/ticket
Templates:: responsive-ticket, jinja2, classic, responsive, minimal, devel
Select template [classic]: classic
Back ends:: anydbm, sqlite
Select backend [anydbm]: sqlite

---------------------------------------------------------------------------
 You should now edit the tracker configuration file:
   /opt/roundup/ticket/config.ini
 ... at a minimum, you must set following options:
   [mail]: domain, host
   [tracker]: web

 If you wish to modify the database schema,
 you should also edit the schema file:
   /opt/roundup/ticket/schema.py
 You may also change the database initialisation file:

   /opt/roundup/ticket/initial_data.py
 ... see the documentation on customizing for more information.

 You MUST run the "roundup-admin initialise" command once you've performed
 the above steps.
---------------------------------------------------------------------------

[austin@support roundup]$ ls
ticket  ticket_backup
[austin@support roundup]$ cd ticket
[austin@support ticket]$ ls
TEMPLATE-INFO.txt  config.ini  detectors  extensions  html  initial_data.py  schema.py  static
[austin@support ticket]$ cp ~/roundup_config.ini config.ini
[austin@support ticket]$ roundup-admin initialise
Enter tracker home: /opt/roundup/ticket
Admin Password:
       Confirm:
Traceback (most recent call last):
  File "/usr/bin/roundup-admin", line 3, in <module>
    run()
  File "/usr/lib/python2.7/site-packages/roundup/scripts/roundup_admin.py", line 49, in run
    sys.exit(tool.main())
  File "/usr/lib/python2.7/site-packages/roundup/admin.py", line 1635, in main
    ret = self.run_command(args)
  File "/usr/lib/python2.7/site-packages/roundup/admin.py", line 1504, in run_command
    return self.do_initialise(self.tracker_home, args)
  File "/usr/lib/python2.7/site-packages/roundup/admin.py", line 526, in do_initialise
    tracker = roundup.instance.open(tracker_home)
  File "/usr/lib/python2.7/site-packages/roundup/instance.py", line 327, in open
    return Tracker(tracker_home, optimize=optimize)
  File "/usr/lib/python2.7/site-packages/roundup/instance.py", line 92, in __init__
    self.backend = backends.get_backend(rdbms_backend)
  File "/usr/lib/python2.7/site-packages/roundup/backends/__init__.py", line 42, in get_backend
    module = __import__(module_name, vars)
ImportError: No module named back_
[austin@support ticket]$ vi /opt/roundup/ticket/initial_data.py
[austin@support ticket]$ python
Python 2.7.5 (default, Apr 11 2018, 07:36:10)
[GCC 4.8.5 20150623 (Red Hat 4.8.5-28)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import sqlite3
>>> exit()
[austin@support ticket]$







------------------------------------------------------------------------------
Check out the vibrant tech community on one of the world's most
engaging tech sites, Slashdot.org! http://sdm.link/slashdot