Cannot release my retail purchased Windows 10 key from a PC I no longer have.

If the old PC is gone and its device entry was deleted, the digital license can no longer be transferred through Activation Troubleshooter. In that situation, the remaining supported options are:

1. Enter the original 25-character product key, if available.
2. If the key was a Microsoft Store purchase, check the purchase confirmation email tied to the Microsoft account used for the purchase. For a Windows 10/11 Pro upgrade bought in the Microsoft Store app, activation uses a digital license rather than a visible product key.
3. If the product key is no longer available, use the Get Help app to contact Microsoft Support for manual activation.

What applies here:

• A substantial hardware change such as replacing the motherboard can cause Windows to lose activation.
• A fresh Windows 11 install on a new drive does not automatically reactivate if the prior device can no longer be identified for transfer.
• Removing the old device from the Microsoft account prevents the troubleshooter from locating the previous installation and transferring the license.

Practical path:

1. On the new PC, open Settings > System > Activation and confirm activation is failing.
2. If the retail purchase included a 25-character key, enter that key.
3. If there is no key and the old device was removed or is no longer available, open Get Help from Start and request Microsoft Support for manual activation.
4. Have proof of purchase ready, because support may need it to verify the retail license.

If the license was used on more devices than allowed, Windows will not activate until a separate valid license is used for the new PC.

References:

• Why can't I activate Windows?
• Find your Windows product key
• Reactivating Windows after a hardware change
• Unable to activate Windows after reinstallation - Microsoft Q&A
• Windows not reactivating. - Microsoft Q&A