Migration from PC to iMac not working
Here is the things you might want to do before wiping the partially set up mac to get back to the automatic migration step described in this other answer


Open the Mac App store and make sure you're signed in to the Apple ID you want to "own" the iLife and iWork apps. Once you've signed in and accepted the license agreement, you can wipe the Mac following this instruction and try the "clean migration" with no user accounts on the Mac to delay/reduce the transfer from PC.
Copy any documents to an external USB drive before erasing things
https://support.apple.com/en-us/HT204904 - choose the erase install and use Internet Recovery to do the erase with no drives connected.


Also, you get phone support with a new Mac. You can do your best on these steps and then call to get a human to make sure you're on the right track with the erase / migration. https://apple.com/support