How to reproduce:
1. Open the attached project
2. Go to Player Settings and expand Icon section
3. Notice that under Adaptive Icons only Foreground layer is attached
4. Make sure that you have SDK 26 and Target API Level is pointing to it
5. Build and Run on Android device
Result: Build fails
Error:
CommandInvokationFailure: Gradle build failed.
/Library/Java/JavaVirtualMachines/jdk1.8.0_121.jdk/Contents/Home/bin/java -classpath "/Applications/Unity 2018.1.0a5/PlaybackEngines/AndroidPlayer/Tools/gradle/lib/gradle-launcher-4.0.1.jar" org.gradle.launcher.GradleMain "-Dorg.gradle.jvmargs=-Xmx2048m" "assembleRelease"

stderr[
/Users/ieva/Downloads/TestIcons/Temp/gradleOut/src/main/res/mipmap-anydpi-v26/ic_launcher.xml:3:35-65: AAPT: No resource found that matches the given name (at 'drawable' with value '@mipmap/ic_launcher_background').

Reproducible: 2018.1.0a5
Note: adaptive icons were not available in previous versions