Thank you for posting your query.
Kindly follow the steps provided below to resolve your issue.
As it says, the correct way of fixing this is as follows:
Backup of <data source name> on <server name> cannot be completed. DPM could not find a valid recovery point on disk.
On the Jobs tab in the Monitoring task area, check for failures of synchronization jobs and resolve those. Next, create a valid recovery point and then run the backup to tape job again. To create a valid recovery point:
1) For application data, in the Protection task area, click Create recovery point, and then select Express full backup on disk.
2) For file data, in the Protection task area, click Create recovery point and then select Create a recovery point after synchronizing.
Go to this link for your reference and other troubleshooting procedures https://social.technet.microsoft.com/Forums/office/en-US/56563da1-0aa2-40c1-a905-8636156e9cc9/dpm-2019-ru1-tape-backup-fails-and-fix-proceedure?forum=dpmtapebackuprecovery
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