Build application in Unity 2017.20f3:

ETC1 compressed textures are not supported when publishing to iPhone
Packages/com.unity.ads@1.0.7/Editor/Resources/Editor/landscape.jpg
Included from scene:
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)
ETC1 compressed textures are not supported when publishing to iPhone
Packages/com.unity.ads@1.0.7/Editor/Resources/Editor/portrait.jpg
Included from scene:
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)
Building - Failed to write file: resources.assets
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)
Build completed with a result of 'Failed'
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)
UnityEditor.BuildPlayerWindow+BuildMethodException: 5 errors
at UnityEditor.BuildPlayerWindow+DefaultBuildMethods.BuildPlayer (BuildPlayerOptions options) [0x00207] in /Users/builduser/buildslave/unity/build/Editor/Mono/BuildPlayerWindowBuildMethods.cs:172
at UnityEditor.BuildPlayerWindow.CallBuildMethods (Boolean askForBuildLocation, BuildOptions defaultBuildOptions) [0x00050] in /Users/builduser/buildslave/unity/build/Editor/Mono/BuildPlayerWindowBuildMethods.cs:83
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)

Solution:

1. Go to:

on MAC
Users/%userprofile%/Library/Unity/cache/packages/packages.unity.com/com.unity.ads@1.0.7/Editor/Resources/Editor/

on Windows
%userprofile%\AppData\LocalLow\Unity\cache\packages\packages.unity.com\com.unity.ads@1.0.7\Editor\Resources\Editor

2. Open portrait.jpg.meta and landscape.jpg.meta
3. Find field textureFormat: -1
4. Change this value to this one textureFormat: 4