UnityAds Mediation not working and Impressions not being counted.

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To get log on iOS, you can follow this guide:

How To - Capturing Device Logs on iOS

For troubleshooting and debugging, it is often very helpful to capture the device logs. The device logs contain information about the status of a running application on the device and any errors encountered. The IAP Engineering team requires logs for open issues. It is assumed here that you have an iOS device attached via USB to your Mac development system. Only Development Builds can be debugged this way, it's not possible to easily view the logs of a released iOS app. You can view the system l…

To get log on Android:

How To - Capturing Device Logs on Android

For troubleshooting and debugging, it is often very helpful to capture the device logs. The device logs contain information about the status of a running application on the device and any errors encountered. The IAP Engineering team requires logs for open issues. It is assumed here that you have an Android device configured with Developer options and attached to a PC with a USB cable. These steps show the process on a Windows system and works similarly on a Mac In Unity, find the location of …

View logs with Logcat  |  Android Studio  |  Android...

You can check the log to see if there are any errors occurred:
https://docs.unity.com/ads/manual/UnityAPI#UnityAdsInitializationError