Thanks for your replying! No the server is not start.. Actually the reason why I need to run
rabbitmqctl enable_feature_flag all was that my rabbitmq-server cannot be started and an error was given: "failed_to_initialize_feature_flags_registry,{rabbit,start, ". After googling I found the reason for this error could be that the feature flags are not started. So do we have to start the server first and then the feature flags can be enabled?