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This is the full error message.

nabil@nabil-Ideapad:~$ roscore
... logging to /home/nabil/.ros/log/bad5fa44-b234-11ed-82e8-9822efcf11bb/roslaunch-nabil-Ideapad-7538.log
Checking log directory for disk usage. This may take a while.
Press Ctrl-C to interrupt
Done checking log file disk usage. Usage is <1GB.

Error processing line 1 of /usr/lib/python3/dist-packages/matplotlib-2.1.1-nspkg.pth:

Fatal Python error: init_import_site: Failed to import the site module
Python runtime state: initialized
Traceback (most recent call last):
  File "/usr/lib/python3.9/site.py", line 175, in addpackage
    exec(line)
  File "<string>", line 1, in <module>
  File "/usr/lib/python3.9/importlib/util.py", line 2, in <module>
    from . import abc
  File "/usr/lib/python3.9/importlib/abc.py", line 17, in <module>
    from typing import Protocol, runtime_checkable
  File "/usr/lib/python3.9/typing.py", line 26, in <module>
    import re as stdlib_re  # Avoid confusion with the re we export.
  File "/usr/lib/python3.9/re.py", line 145, in <module>
    class RegexFlag(enum.IntFlag):
AttributeError: module 'enum' has no attribute 'IntFlag'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/lib/python3.9/site.py", line 605, in <module>
    main()
  File "/usr/lib/python3.9/site.py", line 592, in main
    known_paths = addsitepackages(known_paths)
  File "/usr/lib/python3.9/site.py", line 375, in addsitepackages
    addsitedir(sitedir, known_paths)
  File "/usr/lib/python3.9/site.py", line 214, in addsitedir
    addpackage(sitedir, name, known_paths)
  File "/usr/lib/python3.9/site.py", line 185, in addpackage
    import traceback
  File "/usr/lib/python3.9/traceback.py", line 5, in <module>
    import linecache
  File "/usr/lib/python3.9/linecache.py", line 11, in <module>
    import tokenize
  File "/usr/lib/python3.9/tokenize.py", line 32, in <module>
    import re
  File "/usr/lib/python3.9/re.py", line 145, in <module>
    class RegexFlag(enum.IntFlag):
AttributeError: module 'enum' has no attribute 'IntFlag'
RLException: Invalid <param> tag: Cannot load command parameter [rosversion]: command [['rosversion', 'roslaunch']] returned with code [1]. 

Param xml is <param command="rosversion roslaunch" name="rosversion"/>
The traceback for the exception was written to the log file