As @MrGeneration already said, you probably didn’t configure elasticsearch/zammad the proper way.
You even linked the thread with the proper solution to the problem you’re reffering to:
The documentation you used also states something you most likely didn’t do:
Next, you will need to make Zammad work with Elasticsearch and rebuild index. You can do it with the following command:
zammad run rails r "Setting.set('es_url', 'http://localhost:9200')"
zammad run rake searchindex:rebuild
Please have a look at the official documentation and make sure you did everything as stated there:
https://docs.zammad.org/en/latest/install-elasticsearch.html
cheers