You are using scram 512 right ?

Scram doesn't require serviceName="kafka"; in jaas file.

You can also use below config in server.properties instead of separate jaas file.

listener.name.sasl_ssl.scram-sha-512.sasl.jaas.config=org.apache.kafka.common.security.scram.ScramLoginModule required username="kafkaadmin" password="kafkaadmin123456";

Ref : https://docs.confluent.io/platform/7.0/kafka/authentication_sasl/authentication_sasl_scram.html