Hi

Thanks for contacting us. In general, it's a Joomla warning known to all J! webmasters since Joomla 1.x. It means that the author of the article doesn't exist now. Here are some basic solutions that you can use to troubleshoot and resolve this issue:

Edit each article and in Publishing (tab) > Created By > choose an author from the list of current users > Save & Close (repeat for others articles) Trash all demo articles that you don't need. Create your own articles. Create a new User (User Manager > New) - then using PHPMyAdmin in the prefix_users table change his ID from the current number to the number from the warning message. Remember that any changes to your website should be performed carefully and with proper backups in place to avoid data loss (step 2). If you're unsure about 3rd step, it's better to seek assistance from someone with experience in the Joomla database.

You can follow this link below:

https://www.joomshaper.com/documentation/helix-framework/troubleshooting-faq

In a short:

Please go to Content -> Article and for each article set the admin or any other user in Created By option. It will be fixed.

https://prnt.sc/Ym8v7A2OY-Dq

-Regards.