Hi Elaine,
What is the version of your WSUS server? Have you approved the Window 10 upgrade on the WSUS server?
To upgrade clients to window10, we need to approve window 10 upgrade on WSUS server, and add .esd to MIME Types in IIS. On client, we need to enable OSUpgrade registry key to allow OS upgrade.
Besides, check if the win7 client has installed all updates.
Best Regards,
Anne
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