Based on a support ticket that I've opened on Azure the endpoint above doesn't exist any longer.
As you can see, the URL takes you to a 404 page. https://learn.microsoft.com/en-us/rest/api/appservice/web-apps/copy-production-slot
The alternative mentioned in the support ticket is to use the following commands:
$SrcSlot = Get-AzWebAppSlot -ResourceGroupName "test-rg" -Name "test-app" -Slot "production"
New-AzWebAppSlot -ResourceGroupName "test-rg" -Name "test-app" -SourceWebApp $SrcSlot -Slot "production-clone"