The event description for Event ID 6398 is:

The Execute method of job definition Microsoft.Office.RecordsManagement.Preservation.PreservationJobDefinition (ID a71c3cb6-21e0-46fd-91b5-9d9f0c2170e7) threw an exception. More information is included below.

The search request was unable to connect to the Search Service.

This error shows up in the Application log from the SharePoint Foundation source and typically appears if you don’t have Search running on your SharePoint farm. This is because there is timer job eDiscovery In-Place Hold Processing that runs hourly and leverages the Search Service to do its work. eDiscovery In-Place discovery has been retired and as of July 1, 2020 no new eDiscovery searches can be created in Online only SharePoint installations.

In order to keep this error from showing up simply navigate to Monitoring, click on “Review Job Definition” and disable the above named timer job.