You can install Windows 10 without a key (click "I don't have a product key") – it will run indefinitely with a watermark and limited personalization.
Edition mismatch (trying to use a Home key on Pro install), or hardware changes. Fix: Run slui.exe 4 for phone activation, or reinstall with correct edition. microsoft windows 10 home pro x64 clean iso
Once you have the clean ISO installed, maintain that cleanliness: You can install Windows 10 without a key