A backup job that targets a Dell Data Domain repository completes with the warning, "Immutability flag on backups was set with warning:", specifically with the endings:

Immutability period for some backups exceeds the maximum value allowed by the target repository, using the maximum supported value instead.

or

Immutability period for some backups is lower than the minimum value allowed by the target repository, using the minimum supported value instead.  

This occurs when the immutability period required by the job within Veeam Backup & Replication either exceeds the 'Max retention period' value or is lower than the 'Min retention period' value set for Retention Lock on the target Data Domain storage.

Starting from Veeam Backup & Replication 12.1, backups stored on a Data Domain store can be marked as immutable to prevent accidental or malicious deletion. If the max and min retention period values on the target Data Domain store are not set high and low enough to accommodate a value for the immutability period needed by the job, Veeam will set the immutability period for a file to a corresponding value allowed by the Data Domain store setting, which will result in the warnings described in this KB article.

For example, if the immutability retention Veeam would set is 30 days, but the 'Max retention period' within the Data Domain settings is only 14, Veeam will instead set the immutability to 14 and display the warning:

Immutability flag on backups was set with warning: Immutability period for some backups exceeds the maximum value allowed by the target repository, using the maximum supported value instead.

To address this, check the Retention Lock settings on the Data Domain store and make sure those are set to a value greater and lower than what's needed by the backup job(s) writing to the storage. If the 'Max retention period' setting is too low, increase it to match the required period, if the 'Min retention period' setting is too high, decrease it to match the required period. Then re-run the job(s) to confirm the issue is resolved.
 
Note: When calculating the maximum immutability duration required for Veeam backup jobs, it's important to consider any GFS (Grandfather-Father-Son) restore points that may be created. For example, if long-term retention in the backup job is set for 7 yearly full backups, the 'Max retention period' setting on the Data Domain store has to be set to a value higher than 7 years.