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.
- Open internet explorer and log into your SharePoint site in IE.
- How to Open IE mode in Edge browser.
- Go to this link: edge://settings/defaultBrowser or Click on the Ellipsis […] in the top right-hand corner of the browser window and click on settings, navigate to Default Browser option.
- Update sites to reloaded in IE
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.