Do you try anything or at least try this steps?
1- Force quit Messages app and then reopen it. To do this, choose Force Quit from the Apple menu () in the upper-left corner of your screen.And then select Messages and click Force Quit. The re-open the Messages app.
2- Restart your Mac. Simply click the Apple menu and select Restart.
3- Sign out and then sign back in. Open Messages, then select Messages and Preferences from the Menu bar. The click Sign out.
Then open the Messages app and sign in with your Apple ID.
4- Open the Messages preferences window and select the Accounts tab (Messages > Preferences), select iMessage (left list), make sure that the checkbox “Enable this account” is checked, if not checked, check the box.
5- Make sure that your Mac is connected to the Internet. Try to load a web site to check your Internet connection. If you do not have an Internet connection, you may try some basic troubleshooting tips like restarting your Mac, Router, Wi-Fi spot etc.
6- Your problem may be because of your date and time settings. Lets resolve this. Click Apple menu > System Preferences > and then Date & Time; select the Time Zone tab and the “Set date and time automatically” checkbox, if it is already selected, unselect and wait a few seconds and reselect again. You may need to enter the admin password if the checkbox is dimmed.
7- On your Mac, Messages > Preferences > Accounts > iMessages Account; and on your iOS device, iPhone or iPad > Settings > Messages > Send and Receive the account should be same.
8- The issue may be because of Keychain problem. Here is how you can resolve this issue:
- Launch Keychain Access (Launchpad > Keychain Access) then click File and Lock Keychain “Login” and then again go to File > Unlock Keychain “Login”, enter your admin password when you are asked. This will rebuild your Keychain Access which may fix this problem.
**If you are still having this problem, please contact Apple support.**