Apple advises if your MacBook stops responding and displays the spinning beach ball following creation of a user account in Setup Assistant just wait – the setup process will resume after about 30 minutes.
It’s possible to avoid the delay by completing the Setup Assistant without connecting to the Internet. This includes both wired and wireless connections. If you’re using an Ethernet adapter, disconnect it before you start the setup process. Once the Setup Assistant is finished, restart your Mac, and then join your Wi-Fi network or connect your Ethernet adaptor.
Although Apple is staying Mum on the actual cause of the issue, simply providing a workaround, some reports suggest that the issue is related to account provisioning and possibly iCloud. It wouldn’t be the first time iCloud has caused issues for Apple.