new iMac hung at set up of iCloud account

I'm setting up a new iMac from my 10-year old iMac, which I want to keep as a second computer. All going well until the new computer says it wants to set up the iCloud account. Password entered correctly, but then it just sits on a spinning ball with 'Setting up account' message.

What to do? I'm cautious about just turning off the computer and trying again.