As mentioned in the comments, since its a Modern site, it will not open in SharePoint designer. This is by design.
When you design your solutions, consider these key areas related to
the NoScript setting:
- Sandbox solutions are not supported.
example, via user custom actions).
- You cannot access sites using
Some web parts are not available for end users.
5.Ability to access or update site property bag entries.
Reference - Customizing "modern" team sites
If its a classic site, you need to ensure that you have enabled scripting at the tenant settings. You then either need to wait 24 hours or run the command mentioned below.
Once done, run the following PS command. You need SharePoint admin creds to run:
Connect-SPOService -Url https://yourtenant-admin.sharepoint.com
Set-SPOSite -Identity https://yoursitecollection.sharepoint.com/sites/test -DenyAddAndCustomizePages 0
Also, in your site collection, go to the
https://sitecollectionurl/_layouts/15/SharePointDesignerSettings.aspx and ensure that
Enable SharePoint Designer option is set to true.