"An unknown error occurred"
Trying to sign into iMessage with my Apple ID on my MacBook Air and can't get it to work. Keep getting this error message. Help!
Oh, I love those "unknown" error messages. Rather than making a lot of guesses I usually just toss those to Apple to figure out. ;-)
To contact Apple, click here --> Choose your country or region - Official Apple Support
Select your country, then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.
or:
Contact Apple for support and service by telephone --> Contact Apple for support and service - Apple Support