>> object not found for file system backup: Microsoft SharePoint Resources.....
Info bptm (pid=178037) using 30 data buffers
Error bptm (pid=178037) media manager terminated by parent process
Error bpbrm (pid=177645) could not send server status message to client
Info bpbkar (pid=15112) done. status: 71: None of the files mentioned in the file list exist or may not be accessible
end writing none of the files in the file list exist (71)
All pre-requisites are complete, but the error is still there.
Please refer below article an follow all points, especially point 6 for SharePoint (SP) Backup policy configuration.
Make sure you us have NBU Clients Installed on all servers in the SP farm.
Check the configuration in above article if its SP 2016, as service account configuration is different.
Hope this helps. Please Mark solution if issue is resolved.