When posting a question regarding Enterprise Server, or really any product, always specify the product name, version, and platform. And where you saw the error message in question, and what you did that produced it.
That message text appears to come from MFDS, but in that case it should be prefixed with "MDS2000E". And there should be previous messages with additional information.
Also, it would help to know whether this is a development or production environment, whether you have previously successfully started Enterprise Server on this system, whether anything significant has changed since then (e.g. product or OS updates)....
As Torkel wrote, the one possible explanation is that the SYSTEM account lacks necessary privileges for something. However, from reviewing the source code I believe the most likely explanation for that -15 error code is that the region is already running.