When I did a clean install, the system activated before I even signed into the Microsoft Account. Remember, you do not use a Windows 7 or Windows 8 key, they become useless with Windows 10.
You did have to complete an upgrade on a qualifying system and probably have signed into a Microsoft Account. If the process did not complete, the clean install will not activate. I have had several systems that would not activate in certain circumstances and have never seen that error.