But when we tried to register the integration with the APM server and try to access the APM section, the integration didn't work. The result is an error popup indicating that some indexes are missing and the data cannot be displayed.
I'm following the Helm instruction for the deployment of the APM. Do you suggest doing an extra configuration to integrate it with Kibana? Is this integration not ready out of the box with the Helm Charts(with their default values)?
The Helm Chart for APM is really meant for 7.x, in 8.x there were significant changes that require additional configuration than what the helm chart offers. These instructions are probably better. The APM server that the Helm Chart offers is considered "legacy". If you want a slightly better out of the box experience, I'd recommend looking at the Elastic Fleet & Agent setup, which offers the APM server as an integration, and is "simpler" to get setup.