After updating my MAC OS to latest version, due to some issues, my icloud / apple account logout and I was never able to login after that. I tried many methods shown in Apple Support Forums and nothing helped me.
I was due to release a new version of an App developed for one of our client. I tried contacting Apple Support team and nothing worked.
After searching a lot, I find a solution which made my life easy.
Here is what I have done to fix the problem:
Verification Failed – An unknown error occurred. MAC Catalina
Open Terminal and Type the following commands
sudo -v ; killall -9 accountsd com.apple.iCloudHelper ; defaults delete MobileMeAccounts ; mkdir ~/Library/Accounts/Backup; mv ~/Library/Accounts/*.sqlite* ~/Library/Accounts/Backup/ ; killall -9 accountsd com.apple.iCloudHelper ; sudo reboot