This error will occur when the port used by the td-agent service is used by another service. Hence, you can either change the port used by the td-agent using port configuration, as described in the wiki page or make sure the configured port is not used by any other process.
<source>
@type forward
port 8090
</source>
As part of td-agent troubleshooting process, if you could not conclude the exact cause from the error logs, tune the log’s to Debug mode using the “log_level debug” parameter for more verbose information. For more details on this, do refer to the td-agent wiki page.