To further follow up from Sai deff going in the right direction...

If you are running windows and connecting to ubuntu the broker needs to be accessible from the host machine. This would require either adjustment on the brokers server and or following the steps needed to obtain the correct route. This question has actually been asked before and seems like resolution steps are out there:

• Connect to Kafka running in WSL2 Ubuntu
• running Kafka on WSL and make producer on windows
• Unable to produce to Kafka topic that is running on WSL 2 from Windows

there is a medium article as well that faced the same:

• https://medium.com/@giodegas/running-apache-kafka-message-broker-in-a-wsl2-ubuntu-22-04-41c342391ca6

In summary you could attempt the following:

1. Modify server.properties to listen on 0.0.0.0 and advertise the correct IP address.
2. Ensure the Ubuntu firewall allows connections to port 9092.
3. Update your Java producer to use the correct IP address of the Kafka broker.