A group of Microsoft Products and technologies used for sharing and managing content, knowledge, and applications.
Dear Nacnudddd,
The Tree View feature should work in SharePoint Online. Please check if you enable it via clicking on Site Settings>Navigation Elements under Look and Feel>check Enable Tree View. If no do that to see the result. Meanwhile, check whether the issue happens in a specific site. Welcome to upload some screenshots showing the issue in your environment for our clarification.
Note: Remove any private information before uploading the screenshots.
Best Regards,
Cliff