"Unknown Error" Logging into iCloud on New Mac

[CloneWarsViewer]

I was helping a friend set up a new Mac. It's my friend's first Mac, but my friend already has an Apple ID from having used an iPhone for a while. During the initial set up on the Mac, my friend skipped the WiFi and Apple ID login steps. After the initial set up wizard completed, my friend set up the WiFi connection and was able to load a website in Safari.

However, when my friend tries to log into the App Store or in System Preferences with the Apple ID, the verification code is sent to the iPhone, but when the code is entered into the prompt on the Mac, it says "unknown error".

The Mac was updated to macOS Catalina (it originally had macOS Mojave when it was started up) and the iPhone has iOS 13. My friend is able to log onto the Apple ID website, so the username and password seem to be correct.

Any ideas what the problem could be or ideas how to fix the issue?

 

[CloneWarsViewer]

So, it's finally working now. My friend tried to log into iCloud in System Preferences with the Apple ID again and it worked this time. We're not really sure why or what, if anything, was different this attempt.