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Go to the Settings menu. From there, scroll down and select iCloud. You will be asked to login to your iCloud account – enter your Apple ID and password. Then you will have to verify the terms & conditions.
Please try to login using above steps first and then see if you are still getting that message
I checked the status of iclould server using below link and all are in green right now :-
Also below work around has also helped many uses to get rid of this error message ;-