Once Microsoft validates your key, you will be prompted to select:

If you'd like to move forward with the installation, let me know: Do you have your ready?

To ensure your ISO wasn't corrupted during download, verify its SHA-1 checksum.

Then he remembered: the old trick. His laptop had originally come with Windows 8. Core . Not Pro. He typed a generic install key for Windows 8.1 Core (found on a buried forum post from 2014). It worked.