Could not sign in to iMessage.

when I try to activate iMessge on my MacBook Air, an error message appear as follows

"Could not sign in to iMessage. An error occurred during activation. Try again."

And "Hand off" function also not work.

When I format and reinstall the Mac OS then everything worked perfect but only for 3 4 days.

I am using MacBook Air 13 inches and iPhone 6 plus. Both devices are updated to latest versions.

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