Unity:  2020.3.15 and below,  2021.1.16 and below | Appodeal SDK: 3.0 | Distribution: UPM  

The error appears because the the Unity version that you are running does not have AssetDatabase.SaveAssetIfDirty() method.

The error from Unity Editor Log:

'AssetDatabase' does not contain a definition for 'SaveAssetIfDirty'.

To solve the error you need to update your Unity version to 2020.3.16 or above, or 2021.1.17+ or above.