According to the official documentation, when adding a group or user, in "Group or User", enter a Windows domain user or group account in the following format: <domain><account>. Can you try '<domain>\Test_Group'?
You can refer to this link: https://learn.microsoft.com/en-us/sql/reporting-services/security/grant-user-access-to-a-report-server?view=sql-server-ver16.
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