How to use Barman's passive server?

Question

I followed Barman's document to setup a primary(backup) server and a passive server:

http://docs.pgbarman.org/release/2.10/

• node1: backup server(10.0.1.1)
• node2: passive server(10.0.1.2)

On node2, I set /etc/barman.d/passive-server.conf as:

[passive]
description =  "Barman passive server"
primary_ssh_command = ssh barman@backup

Then switch to barman user to test:

-bash-4.2$ barman sync-backup
usage: barman sync-backup [-h] server_name backup_id
barman sync-backup: error: too few arguments

Because I set PostgreSQL server info on the node1 as:

/etc/barman.d/app_db-server.conf

[app_db]
description = "app_db PostgreSQL server"
conninfo = host=app_db user=barman dbname=postgres
streaming_conninfo = host=app_db user=streaming_barman dbname=postgres
backup_method = postgres
streaming_archiver = on
slot_name = app_db

But didn't config /etc/barman.d/passive-server.conf on the node1.

I tried to run on node2 again:

-bash-4.2$ barman sync-backup app_db 20200311T121411
ERROR: server app_db is not passive

If a I rm barman cron on node1, I found this directory has been created and some data was there on node2:

$ sudo ls /var/lib/barman/
app_db streaming-app_db

However, in the app_db folder, only 2 folders found: streaming wals. In the streaming folder, there is nothing.

Is the usage wrong?