If you are having issues building your project in the Unity Editor for Android, ensure that your Android SDK minimum version is set to 5.0 and then build again.
If the Android build continues to fail, the issue is likely that there are some conflicting dependencies in your game. For example, if you are using an SDK version with a dependency that is incompatible with Unity Mediation.
In this case, try updating the version of the other SDK config file before you build again. If the Android build fails persist, contact Unity Support or the Open Mediation SDK team for help.
For more information, see the Unity Mediation documentation.