Not able to open a SharePoint modern site using SharePoint Designer

VigneshGanesan There is a permission you have to set through PowerShell to allow SharePoint Designer to connect, and to get rid of that error message you see. You have to do it for every site where you want to use SPD.

$cred = Get-Credential

Connect-SPOService -Url https://TENANTNAME-admin.sharepoint.com -credential $cred
Set-SPOSite -Identity https://TENANTNAME.sharepoint.com/sites/SITENAME -DenyAddAndCustomizePages $false

# to test if it changed
$site = Get-SPOSite -Identity https://firemountain.sharepoint.com/sites/Office365Training
$site.DenyAddAndCustomizePages

Disconnect-SPOService