Try Travis' solution

.travis.yml

@@ -10,6 +10,8 @@ android:
     # The SDK version used to compile NewPipe
     - android-27
 
+before_install: 
+  - yes | sdkmanager "platforms;android-27"
 script: ./gradlew -Dorg.gradle.jvmargs=-Xmx1536m assembleDebug lintDebug testDebugUnitTest
 
 licenses: