It's not actually a problem, just a warning that can seem alarming to new users. A NIS (Network Information System) allows "a group of machines within an NIS domain to share a common set of configuration files." Basically, if you're running email servers on more than one box, you can share config files between them.
In this case, since you are just running a single server, we can remove the line causing this error.
If we run `postconf | grep nis` we can see that the postfix configuration utility has following line:
alias_maps = hash:/etc/aliases, nis:mail.aliases
You can take out the `nis:mail.aliases` part by running this command:
postconf -e "alias_maps = hash:/etc/aliases"
This will remove the warnings from your logs. You will also need to restart your Postfix service for the changes to take effect:
service postfix restart