After some tests....,the error ' port 602 is not available.' should be different ( access denied).
In my case, the access to this port is denied due to the fact that splunk is running in non root user and 0-1024 port are not available for non root listen socket.
root@srv-splunk:/opt/splunk/bin# ./splunk add tcp 602
Splunk username: admin
Password:
Login failed
root@srv-splunk:/opt/splunk/bin# ./splunk add tcp 602
Splunk username: admin
Password:
In handler 'raw': Parameter name: TCP port 602 is not available.
I have also try via GUI but without success ?