Hi @Chinzorig, Tsomirlog (Ulaanbaatar),
First ensure you have the necessary permissions to make changes to the list settings. You need to be an admin or have full control permissions for the list.
Then you can clear your browser cache or using a different browser. Sometimes browser compatibility issues can cause the SharePoint page to fall back to the classic experience.
Finally check if there are any list-level customizations, scripts, or web parts that might be forcing the classic experience. If you have access to SharePoint Online Management Shell, you can use PowerShell commands to enforce the modern experience. For example:
Connect-SPOService -Url https://yourdomain-admin.sharepoint.com
Set-SPOSite -Identity https://yourdomain.sharepoint.com/sites/yoursite -DenyAddAndCustomizePages 0
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