I have already installed "six" 1.7.2 in python 2.7.18 with pip, but i receive an error message anyway.

seva@seva-HLYL-WXX9:~$ python --version
Python 2.7.18

seva@seva-HLYL-WXX9:~$ pip install six==1.7.2
DEPRECATION: Python 2.7 reached the end of its life on January 1st, 2020. Please upgrade your Python as Python 2.7 is no longer maintained. pip 21.0 will drop support for Python 2.7 in January 2021. More details about Python 2 support in pip can be found at https://pip.pypa.io/en/latest/development/release-process/#python-2-support pip 21.0 will remove support for this functionality.
Requirement already satisfied: six==1.7.2 in ./.pyenv/versions/2.7.18/lib/python2.7/site-packages (1.7.2)

seva@seva-HLYL-WXX9:~$ cqlsh
Traceback (most recent call last):
  File "/usr/bin/cqlsh.py", line 130, in <module>
    from six.moves import configparser, input
ModuleNotFoundError: No module named 'six.moves'

seva@seva-HLYL-WXX9:~$ pip show six
DEPRECATION: Python 2.7 reached the end of its life on January 1st, 2020. Please upgrade your Python as Python 2.7 is no longer maintained. pip 21.0 will drop support for Python 2.7 in January 2021. More details about Python 2 support in pip can be found at https://pip.pypa.io/en/latest/development/release-process/#python-2-support pip 21.0 will remove support for this functionality.
Name: six
Version: 1.7.2
Summary: Python 2 and 3 compatibility utilities
Home-page: http://pypi.python.org/pypi/six/
Author: Benjamin Peterson
Author-email: benjamin@python.org
License: MIT
Location: /home/seva/.pyenv/versions/2.7.18/lib/python2.7/site-packages
Requires: 
Required-by: thrift

Answer

If you want to use python2.7, open cqlsh.py with nano:

sudo nano /usr/bin/cqlsh.py

And change python3 on python2.7.