How can I test block download policies in SPO?

You’re trying to verify how CSOM File.OpenBinaryStream() behaves when a SharePoint site has Block Download Policy enabled, but your developer tenant will not offer the SharePoint Advanced Management trial. This policy requires SharePoint Advanced Management prerequisites, so a developer subscription cannot reproduce the supported configuration if that entitlement is unavailable. Use a licensed test tenant, create an isolated site and file, then enable the policy with Set-SPOSite -Identity <site> -BlockDownloadPolicy $true. Run the same CSOM code as a normal delegated user who can read the file, both before and after the policy applies. File.OpenBinaryStream() requests file content, so capture the resulting ClientRequestException or ServerException, status details, and correlation ID rather than assuming a particular error. Do not test with app-only authentication: Microsoft documents that app-only service applications are exempt. Without the required entitlement, there is no supported way to emulate this policy in that tenant