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First of all you need your Java class available on the server. Usually, this is achieved by uploading the jar to artefact repo in business central and adding a dependency to this jar in your project (but other alternatives like adding it to your project itself, or adding it to the server classpath are possible as well).
Second of all, you need to register a handler for "Service Task", you could do this at the server level, by editing
"Service Task" : new org.jbpm.process.workitem.bpmn2.ServiceTaskHandler(ksession)
The error you are seeing seems to be because the second step seems to be missing.