Seems that your Agent is not yet received the machine GUID from the server. It is OK for cases when the Agent is installed not from Notification Server directly. For example by the pull method or by some re-imaging job. In this case the Agent send registration request to the Notification Server (later NS). The NS should then assign the unique identifier to the Agent (the GUID) which could be seen from the Agent Settings page (when received).
In your case it is now depends on the NS when your agent will receive it. If you have a big timeout for configuration update, you could try to perform it manually by clicking "Update Configuration". This will just re-request the registration. If it does not help, then need to check the NS for details regarding your agent, whether it was not blacklisted. If not then update required tasks on NS in order to speed up the registration process.