Conduktor only fully support ksqlDB from version 0.10.x. Most things works with older versions except the ksqlDB queries (you may end up with errors like KsqlClientException: Received 404 response from server: HTTP 404 Not Found. Error code: 40400)
If you're using Docker, double-check which Docker image you're using. Confluent Platform 5.5.1 is only using ksqlDB version 0.7.1 which is very old (many stuff changed since), you can refer to their compatibility matrix here: https://docs.confluent.io/current/installation/versions-interoperability.html#ksqldb. ksqlDB 0.10.x will be supported only for Confluent Platform 6.x.
If you encounter an issue, you may instead use the Docker image
confluentinc/ksqldb-server:0.11.0 as described in the official quickstart: https://ksqldb.io/quickstart.html