In the modern team site in SharePoint navigation section is come by default, If you want to hide the left navigation in the modern team site.
To hide the section, follow the steps.
2. After clicking on the gear icon, click on the site information.
3. On clicking the site information, "edit site information" box appears. Scroll down and click on the View all site settings.
4. On clicking the view all site settings, site settings appear, click on the Navigation Elements.
5. On clicking Navigation Elements, uncheck the options.
6. Uncheck the options and click on OK.
7. Now the Left navigation is disappeared from your site.