Barman Backup failed issuing start backup command

PostgreSQL
1 
barman backup pg
$ barman list-server
pg - Backup PostgreSQL Database (via SSH)
---
$ barman check pg
Server pg:
        PostgreSQL: OK
        superuser or standard user with backup privileges: OK
        wal_level: OK
        directories: OK
        retention policy settings: OK
        backup maximum age: OK (no last_backup_maximum_age provided)
        backup minimum size: OK (0 B)
        wal maximum age: OK (no last_wal_maximum_age provided)
        wal size: OK (0 B)
        compression settings: OK
        failed backups: OK (there are 0 failed backups)
        minimum redundancy requirements: OK (have 0 backups, expected at least 0)
        ssh: OK (PostgreSQL server)
        ssh output clean: FAILED (the configured ssh_command must not add anything to the remote command output)
        systemid coherence: OK (no system Id stored on disk)
        archive_mode: OK
        archive_command: OK
        continuous archiving: OK
        archiver errors: OK

when start backup it failed

$ ```
barman backup pg
ERROR: Impossible to start the backup. Check the log for more details, or run ‘barman check pg’

2

and this is log
Command WARNING: This service is restricted to authorized users only. All activities on this system are logged.
barman.server ERROR: Check ‘ssh output clean’ failed for server ‘pg’
[1431397] barman.server ERROR: Impossible to start the backup. Check the log for more details, or run ‘barman check pg’