Do you know if the machine had been upgraded to windows 10 in the past? If it has been, all you need to do is download the windows 10 image from Microsoft (pro or home and 64 bit of course) if you know what version of 7 it had, makes it easier. make a bootable USB stick with the win10 image (you tube any of this you don't know) don't try to put in a code for it.
When your installed and online, if your computer shows activated for the windows, your good. if not activated, hit troubleshoot activation. will tell you if there is a licence for pro or home. Trying this doesn't cost you anything but time.