Hi Kassidy,
This is Joomla Message known for all webmasters since Joomla 1.x - it's so easy to fix it.
You have to edit all articles and check which one doesn't have author from the list of users, then choose it.
This is task for you to add missing author in Publishing > Created By.
If you don't use those articles, just trash them. Remove all not used articles.
Si vous n'utilisez pas ces articles "de démonstration", supprimez-les. Supprimez tous les articles non utilisés et ajoutez les vôtres. Cela résoudra également votre problème :)
https://www.joomshaper.com/documentation/helix-framework/troubleshooting-faq#juser-load-could-not-load-this-user-user-id-xx