The first time Splunk is installed, it will use the 'Enterprise Trial' license that is bundled with the download package. This is usually valid for between 30 and 60 days.
For forwarders, you generally don't need an indexing capability, so we have also included a 'Forwarding license' in the download package. This is a 1MB, perpetual Enterprise license that will enable all features, like security, distributed search and deployment server, but will not all for any indexing. You can also use this license on search head instances.
Lastly, there is the perpetual, 500MB, free license. You can apply this to your forwarders also, and they will work just fine if all you want to do is forward data, but none of the other features will be enabled - the most important of which is security.
To resolve the messages you are seeing, simply update the $SPLUNK_HOME/etc/splunk.license file with either the free or the forwarder license, and recycle your instances.