
To open SharePoint designer for online sites where modern authentication is enabled, please flow the below steps.
Update following registry entries in your system when SharePoint Designer needs to open.
4. Navigate to any list and open the list in classic mode.
5. Reload the list view page (.aspx page) in IE mode.
6. Reload the list view page (.aspx page) in IE mode.
7. Click on Edit list which will open list and site with designer.