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Hi Nigel,
If Windows 11 Pro was previously activated on that device, a reinstall normally reactivates automatically once the PC connects to the internet. If the product key is no longer accepted, the most common causes are installing a different edition than the original license (for example, Windows 11 Home vs. Pro), using an OEM key on different hardware, or a significant hardware change that broke the digital license association.
First, confirm that Windows 11 Pro was installed and that it matches the edition originally licensed for the device. If you signed in with a Microsoft account before the reinstall, try running the Activation Troubleshooter under Settings > System > Activation and select "I changed hardware on this device recently" if applicable.
If the key still fails, please provide the exact activation error code (such as 0xC004F050, 0xC004C003, etc.) and whether the license is Retail, OEM, or Volume/KMS. The error code will identify whether the issue is an invalid key, edition mismatch, activation limit, or licensing problem.
Harry.