can't connect to the account "icloud"

Can you receive e-mails? One option is to delete the account and add it back.

Mail (El Capitan): Mail troubleshooting

Mail (El Capitan): If you can’t receive messages

Mail (El Capitan): If you can’t send messages

Mail not sending or receiving messages after updating OS X

Enter in user library see this article macOS Sierra: Library folder

Delete the complete contents of folders Caches , Cookies , Saved application state into the trash , restart the Mac and empty the trash .

To work your I cloud mail account you must have to create a web mail address like gmail or any and Apple ID that are kept as same , open system preferences > iCloud account under your name Apple ID must be shown that you must be signed in with I cloud account .

In internet accounts > click on iCloud check all the boxes except back to my Mac and click on web mail account check all the boxes .

See some articles Sign in with your Apple ID - Apple Support

macOS Sierra: Set up iCloud on your Mac

Make sure you are not using any anti virus , third party apps , softwares in your Mac , you can use https://etrecheck.com/

And it will show what is running in the Mac , also use Antivirus Replacement for Mac - Scan and Remove Malware | Malwarebytes

and if any malware or adwares are installed that will be removed .

Try these methods it is possible the Mac is malfunctioning in that scenario a new user account is created see this article How to test an issue in another user account on your Mac - Apple Support

See this article Get help using iCloud Mail - Apple Support

You are welcome. The suggestions are valid for 10.13.