You can only clean install Win10 on the same PC as you activated the Win10 upgrade version from previous Win7/8/8.1. Yes, you must do the upgrade and activate Win10 first then clean install.
Windows10 kinda remembers that you have Win10 already and will let you clean install.
Clean install will not work on a different PC, even if you have activated it before.
Info:
In some countries, not allowing you to install/move your OS/software to another hardware e.g. the case for OEM version is illegal even for Microsoft and thus you can still activate your Windows via calling local Microsoft service center, if you wanna install Windows again on a different PC.
You might wanna try the activation via telephone and see if it works, if not, just buy a new key.
Prepare your key, etc. before you contact the service center.