Q: Cannot activate iMessage on MacPro

My MacPro has iMessage, but it's not activated.  When I try to, it gets as far as me putting in my Apple ID and the spinner just goes for awhile.

Finally, message comes up with "cannot connect, check your system", or something like that.  My wi-fi is working fine.

I've had our local Mac expert here to help and he's mystified.

Any ideas?

Thanks.