eDiscovery Content Search for Items in Purge Folder

To get this to work, you need to provide the identifier for the Purges folder, not its name. The process is covered in this article: https://learn.microsoft.com/en-us/purview/ediscovery-use-content-search-for-targeted-collections

Keep in mind that the identifier will be different for each mailbox, so if you have multiple users to cover, you have to repeat the process for each user. Also, the Purges folder within the Online archive mailbox will have a different id from the Purges folder of the main mailbox, so make sure you provide the correct one.