As an employer or an administrator, you are only able to message a learner if a systems notification has been sent to them, or if the learner contacts you in the first instance.
It is expected behaviour that if there have not been any notifications sent, or if the line manager has been changed, then the employer (or any administrator for that matter) cannot start a conversation.
For our full list of system notifications, please refer to this guide: Aptem system email notifications behaviour.