iMessage and Facetime Activation Errors

After repeated attempts, over several days, unable to activate iMessage and FaceTime on my 2017 MacBook Air 13" running Mojave version 10.14.3. Can access internet and other Apple applications with my Apple ID and password. Have followed "If you can't sign in to iMessage account or FaceTime on your Mac" and "If you get error when trying to activate iMessage or Facetime" guidances to no avail. Message on attempts to activate iMessage: "an error occurred during authentication." Message on attempt to activate FaceTime: "an error occurred during activation, try again." Any advice and guidance to identify and resolve my problems greatly appreciated!