Terminal does not start in Ubuntu 18.04

I tried to install Django Framework in Ubuntu. and i am using python 3.8. but in normal instalation, Django 1.1 had been installed. and i changed the python3 alternative to python3.8 . and usuing python3 -m pip install django. and i installed Django(v 3.3) and used it. but after the changing the alternative, the Terminal doesn't open and start.

i deleted python3.6 from my ubuntu. and the problems had been started after that.

it seems some programs in linux run in python3.6 and after deleting that,

i can't fix the problems and the Terminal windows does not open. i tried other ways (changing the gnome-terminal file, changing the alternative to default v3.6 and fix the Broken distro), but they didn't help me. i can't open Terminal, and when i ran sudo apt update i recieved this Errors:

Any idea?

      File has unexpected size (1133 != 1128). Mirror sync in progress? [IP: ***.**.***.** 80]   
 Release file created at: Wed, 06 May 2020 19:27:45 +0000 Fetched 1,762 B in 2s (932 B/s)  Traceback (most recent call last):   File "/usr/lib/cnf-update-db", line 8, in <module>
        from CommandNotFound.db.creator import DbCreator   File "/usr/lib/python3/dist-packages/CommandNotFound/db/creator.py", line 7, in <module>
        import sqlite3   File "/usr/lib/python3.6/sqlite3/__init__.py", line 23, in <module>
        from sqlite3.dbapi2 import *   File "/usr/lib/python3.6/sqlite3/dbapi2.py", line 23, in <module>
        import datetime   File "/usr/lib/python3.6/datetime.py", line 8, in <module>
        import math as _math ModuleNotFoundError: No module named 'math' Error in sys.excepthook: Traceback (most recent call last):   File "/usr/lib/python3/dist-packages/apport_python_hook.py", line 63, in apport_excepthook
        from apport.fileutils import likely_packaged, get_recent_crashes   File "/usr/lib/python3/dist-packages/apport/__init__.py", line 5, in <module>
        from apport.report import Report   File "/usr/lib/python3/dist-packages/apport/report.py", line 12, in <module>
        import subprocess, tempfile, os.path, re, pwd, grp, os, time, io   File "/usr/lib/python3.6/subprocess.py", line 136, in <module>
        import _posixsubprocess ModuleNotFoundError: No module named '_posixsubprocess'

    Original exception was: Traceback (most recent call last):   File "/usr/lib/cnf-update-db", line 8, in <module>
        from CommandNotFound.db.creator import DbCreator   File "/usr/lib/python3/dist-packages/CommandNotFound/db/creator.py", line 7, in <module>
        import sqlite3   File "/usr/lib/python3.6/sqlite3/__init__.py", line 23, in <module>
        from sqlite3.dbapi2 import *   File "/usr/lib/python3.6/sqlite3/dbapi2.py", line 23, in <module>
        import datetime   File "/usr/lib/python3.6/datetime.py", line 8, in <module>
        import math as _math ModuleNotFoundError: No module named 'math' Reading package lists... Done E: Failed to fetch http://dl.google.com/linux/chrome/deb/dists/stable/main/binary-amd64/Packages.gz File has unexpected size (1133 != 1128). Mirror sync in progress? [IP:
    * 80]    Hashes of expected file:
        - Filesize:1128 [weak]
        - SHA256:*
        - SHA1:*
        - MD5Sum:*
Release file created at: Wed, 06 May 2020 19:27:45 +0000 E: Some index files failed to download. They have been ignored, or old ones used instead. E: Problem executing scripts APT::Update::Post-Invoke-Success 'if /usr/bin/test -w /var/lib/command-not-found/ -a -e /usr/lib/cnf-update-db; then /usr/lib/cnf-update-db > /dev/null; fi' E: Sub-process returned an error code

and below result showed when i tried to run django-admin:

Traceback (most recent call last):
  File "/usr/bin/django-admin", line 5, in <module>
    from django.core.management import execute_from_command_line
  File "/home/pytm/.local/lib/python3.6/site-packages/django/__init__.py", line 1, in <module>
    from django.utils.version import get_version
  File "/home/pytm/.local/lib/python3.6/site-packages/django/utils/version.py", line 1, in <module>
    import datetime
  File "/usr/lib/python3.6/datetime.py", line 8, in <module>
    import math as _math
ModuleNotFoundError: No module named 'math'
Error in sys.excepthook:
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/apport_python_hook.py", line 63, in apport_excepthook
    from apport.fileutils import likely_packaged, get_recent_crashes
  File "/usr/lib/python3/dist-packages/apport/__init__.py", line 5, in <module>
    from apport.report import Report
  File "/usr/lib/python3/dist-packages/apport/report.py", line 12, in <module>
    import subprocess, tempfile, os.path, re, pwd, grp, os, time, io
  File "/usr/lib/python3.6/subprocess.py", line 136, in <module>
    import _posixsubprocess
ModuleNotFoundError: No module named '_posixsubprocess'

Original exception was:
Traceback (most recent call last):
  File "/usr/bin/django-admin", line 5, in <module>
    from django.core.management import execute_from_command_line
  File "/home/pytm/.local/lib/python3.6/site-packages/django/__init__.py", line 1, in <module>
    from django.utils.version import get_version
  File "/home/pytm/.local/lib/python3.6/site-packages/django/utils/version.py", line 1, in <module>
    import datetime
  File "/usr/lib/python3.6/datetime.py", line 8, in <module>
    import math as _math
ModuleNotFoundError: No module named 'math'
profiling:/home/user/Downloads/Python-3.6.10/Programs/python.gcda:Cannot open
profiling:/home/user/Downloads/Python-3.6.10/Modules/xxsubtype.gcda:Cannot open