WAL archive: FAILED (please make sure WAL shipping is setup) - postgresql

Question

I am trying to configure Barman to backup. When I do a barman check replica I keep getting:
Server replica:
WAL archive: FAILED (please make sure WAL shipping is setup)
PostgreSQL: OK
superuser: OK
wal_level: OK
directories: OK
retention policy settings: OK
backup maximum age: FAILED (interval provided: 1 day, latest backup age: No available backups)
compression settings: OK
failed backups: OK (there are 0 failed backups)
minimum redundancy requirements: FAILED (have 0 backups, expected at least 2)
ssh: OK (PostgreSQL server)
not in recovery: FAILED (cannot perform exclusive backup on a standby)
archive_mode: OK
archive_command: OK
continuous archiving: OK
archiver errors: OK
I am using Postgresql 9.6 and barman 2.1; I am not sure as to what the issue is could someone help?
Here is my Barman server configuration:
description = "Database backup"
conninfo = host=<db-ip> user=postgres dbname=db
backup_method = rsync
ssh_command = ssh postgres#<db-ip>
archiver = on

Answer 1

barman check tries to confirm that archiving is set up correctly by asserting that there's actually something in the archive. However, WAL segments are generally only archived once they're filled up, and if your server is idle, this is never going to happen.
To work around this, Barman provides a command to force a segment switch, wait for the completed WAL to show up, and then archive it immediately:
barman switch-xlog --force --archive replica

Answer 2

in brief
Barman's incoming_wals_directory and Postgresql.conf's archive_command not matched as described in details here
details
Another cause is that the not matched between
Barman's incoming_wals_directory
Postgresql.conf's archive_command
Bash util to check
barman#backup $ barman show-server pg | grep incoming_wals_directory
# output1
# > incoming_wals_directory: /var/lib/barman/pg/incoming
postgres#pg $ cat /etc/postgresql/10/main/postgresql.conf | grep archive_command
# output2
# > archive_command = 'rsync -a %p barman#staging:/var/lib/barman/pg/incoming/%f'
We must have same path in :output1 and :output2
Make them matched if they don't and don't forget to restart postgres afterward.