Error to serialize message when sending to kafka topic

I'd say the error is obvious:

Can't convert value of class org.springframework.messaging.support.GenericMessage to class org.apache.kafka.common.serialization.StringSerializer specified in value.serializer

Where your value is GenericMessage, but StringSerializer can work only with strings.

What you need is called JavaSerializer which does not exist, but not so difficult to write:

public class JavaSerializer implements Serializer<Object> {

    @Override
    public byte[] serialize(String topic, Object data) {
        try {
            ByteArrayOutputStream byteStream = new ByteArrayOutputStream();
            ObjectOutputStream objectStream = new ObjectOutputStream(byteStream);
            objectStream.writeObject(data);
            objectStream.flush();
            objectStream.close();
            return byteStream.toByteArray();
        }
        catch (IOException e) {
            throw new IllegalStateException("Can't serialize object: " + data, e);
        }
    }

    @Override
    public void configure(Map<String, ?> configs, boolean isKey) {

    }

    @Override
    public void close() {

    }

}

And configure it for that value.serializer property.

private void configureProducer() {
Properties props = new Properties();
props.put("key.serializer",
        "org.apache.kafka.common.serialization.StringSerializer");
props.put("value.serializer",
        "org.apache.kafka.common.serialization.ByteArraySerializer");

producer = new KafkaProducer<String, String>(props);

}

This will do the job.