Hi, I'm Greg, an installation specialist, 10 year Windows MVP, and Volunteer Moderator. If you will work with me I will be here to help until the issue is resolved.
If the old install was Activated as a Digital License tied to your MS Account, then you can sign into the MS Account on the new Windows 10 and run the Activation Troubleshooter to see if it offers to move the activation to the new install.
If not you'll need to contact Microsoft Support on weekdays by one of these phone numbers:
https://support.microsoft.com/en-us/help/405170...
You can also go here: https://partner.support.services.microsoft.com/... choose the link for Browser help, interact briefly about your exact problem. If you keep saying it didn't help it will connect you with an agent.
I hope this helps. Feel free to ask back any questions and keep me posted. If you will wait to choose if I resolved your problem, I will keep working with you until it's resolved.