I understand that when trying to sign in to iCloud, you get an authentication error. I'm happy to provide some help for this issue.
Check out the troubleshooting provided for this issue in the article below:
Get help with iCloud - Apple Support
I get an “authentication failed” message when I try to sign in with my Apple ID during iCloud setup on my iPhone, iPad, or iPod touch
Go to your Apple ID account page and try to sign in. If you can't sign in, follow the steps provided there to reset your Apple ID account password.
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