Hello there,
The first suggestion would be to Update to the latest rollup for DPM 2019 and update your Windows Server to the latest cumulative update.
The next thing that you should do is verify that the DNS name resolution is working properly for both the DPM server and the target computers. Even if the DNS name resolution seems to be working properly, you should compare the IP address that is returned during the name resolution test with the actual IP address that the target computer is using.
The below thread discusses the same issue and you can try out some troubleshooting steps from this and see if that helps you to sort the Issue. https://social.technet.microsoft.com/Forums/en-US/3a0e62aa-7a46-455f-9a1a-06c79c15a67d/dpm-2016-error-40009-the-specified-operation-requires-the-associated-storage-to-be-mounted-first?forum=dpmstorage
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