Open the "mtaserver.conf" file which is located in your server folder (\MTA San Andreas\server\mods\deathmatch\).
Add the following line in "mtaserver.conf" under the text underneath.
<!-- Specifies resources that are loaded when the server starts and/or which are protected from being stopped.
To specify several resources, add more <resource> parameter(s). -->
<resource src="admin" startup="1" />
Now restart your server and try to register again.