@James As you mentioned, the server was created on March 11th, during which you should have enabled the feature since it was in preview. However, if you create a new server, you should be able to add the private endpoint. If, at that moment, the private endpoint feature was not enabled on the subscription, it would be impossible to add a private endpoint. When the feature was in public preview, a prerequisite was to add the feature to your subscription.For the existing server, you have to wait until a new deployment is made. Currently, we don't have any ETA on when that will be available.
Please let me know if you need any additional information.
Regards
Geetha