Have you setup from Settings --> Filestore --> FTP
Then From Backup you have to select Store backup in FTP
rsync: write failed on “/var/spool/asterisk/tmp/backup-4/var/spool/asterisk/monitor/SIP-6126-00006b4c-2023-08-14-09-56-06.wav”: No space left on device (28)
Your FreePBX disk space run out.