Cmd.exe exited with code '5' on Azure DevOps when multiple python test scripts runs in one go

I'm trying to execute multiple python scripts within Azure DevOps pipeline but the "##[error]Cmd.exe exited with code '5'." is returned with no identification of any testcase apparently.

In the "execute script" step of the pipeline, the following command has been written:

 "C:\Program Files\Python\Python38\python.exe"  -m pytest $(System.DefaultWorkingDirectory)\autotest_main.py -v --junitxml=test-results.xml --html=report.html --self-contained-html

and this works fine untile the script "autotest_main.py" contains just the suite with the test cases. But I need to organize a lot of test cases, so the starting point is the autotest.py script that is written as follows:

import os

os.system('test_Preconditions.py')
os.system('test_Suite1.py')
os.system('test_Suite2.py')
os.system('test_Suite3.py')
os.system('test_Suite4.py')

I also used the test_ notation to make the testcases easy to be identified. Each of the test_Suite1.py file contains than the unit tests in conjunction with selenium webdriver to test the frontend of a web application. When I run the script on the local machine everything works fine but when I run it using azure devops the "##[error]Cmd.exe exited with code '5'." is returned. Here the log:

##[section]Starting: execute script
==============================================================================
Task         : Command line
Description  : Run a command line script using Bash on Linux and macOS and cmd.exe on Windows
Version      : 2.176.1
Author       : Microsoft Corporation
Help         : https://docs.microsoft.com/azure/devops/pipelines/tasks/utility/command-line
==============================================================================
Generating script.
Script contents:
"C:\Program Files\Python\Python38\python.exe"  -m pytest C:\agent\_work\12\s\autotest_main.py -v --junitxml=test-results.xml --html=report.html --self-contained-html
========================== Starting Command Output ===========================
##[command]"C:\Windows\system32\cmd.exe" /D /E:ON /V:OFF /S /C "CALL "C:\agent\_work\_temp\b653526f-53a0-4ac7-8a22-50698ed4ba58.cmd""
============================= test session starts =============================
platform win32 -- Python 3.8.3, pytest-6.1.0, py-1.9.0, pluggy-0.13.1 -- C:\Program Files\Python\Python38\python.exe
cachedir: .pytest_cache
metadata: {'Python': '3.8.3', 'Platform': 'Windows-10-10.0.14393-SP0', 'Packages': {'pytest': '6.1.0', 'py': '1.9.0', 'pluggy': '0.13.1'}, 'Plugins': {'html': '2.1.1', 'metadata': '1.10.0', 'nunit': '0.6.0'}}
rootdir: C:\agent\_work\12\s
plugins: html-2.1.1, metadata-1.10.0, nunit-0.6.0
collecting ... collected 0 items
 
---------- generated xml file: C:\agent\_work\12\s\test-results.xml -----------
--------- generated html file: file://C:\agent\_work\12\s\report.html ---------
============================ no tests ran in 0.32s ============================
##[error]Cmd.exe exited with code '5'.
##[section]Finishing: execute script

Any suggestion to fix this problem? I want to possibly prevent to write a lot of execute script steps within the Azure Pipeline if possible.