If you see the following errors in your project:
GameID not found
Warning, failed to retrieve GameID from Dashboard
This means that the Editor was not able to fetch your Game ID. The most common reason for this error is due to your Unity Dashboard project not being linked to the project inside the Editor.
To resolve this issue, in the Unity Editor go to Edit > Project Settings > Services to check the status of your linked project. Once your project is successfully linked, try building again.
For more information, see documentation on Unity Mediation.