This is the complete error message I get with redacted ID's:
Properties.CanonicalName is invalid. Found a duplicate certificate with {domain_name} available or in pending issued under serverFarmId /subscriptions/{subscription_id}/resourceGroups/{resource_group}/providers/Microsoft.Web/serverfarms/{App_Plan_name}. Pending certificate operation id: {operation_id}, timeout: 7/7/2021 3:48:53 PM.
I have waited for the timeout, but it has been 1 hour past it and still I am not able to try again to create the App Service Managed Certificate.