JupyterHub is started but not able to log in: Error loading server extension jupyterlab

[Raymond Xie]

I am having error below when I tried to use jupyterhub:

Spawning jupyterhub-singleuser --port=50928

[W 2018-09-13 22:41:17.283 SingleUserNotebookApp notebookapp:1554] Error loading server extension jupyterlab

    Traceback (most recent call last):

      File "/usr/lib/python3.6/site-packages/notebook/notebookapp.py", line 1546, in init_server_extensions

        mod = importlib.import_module(modulename)

      File "/usr/lib64/python3.6/importlib/__init__.py", line 126, in import_module

        return _bootstrap._gcd_import(name[level:], package, level)

      File "<frozen importlib._bootstrap>", line 994, in _gcd_import

      File "<frozen importlib._bootstrap>", line 971, in _find_and_load

      File "<frozen importlib._bootstrap>", line 953, in _find_and_load_unlocked

    ModuleNotFoundError: No module named 'jupyterlab'

Why there is error relates to jupyterlab?

I have previous jupyter (with three kernels) on this node, would that matter? 

How can I completely uninstall the existing jupyter related tool?

Thank you very much.

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Sincerely yours,

Raymond