I get this when I set my automationName to UiAutomator2:
Original error: Could not sign with default certificate. Original error Command ‘/usr/lib/jvm/java-8-oracle/bin/java -jar /usr/local/lib/node_modules/appium/node_modules/appium-adb/jars/sign.jar /usr/local/lib/node_modules/appium/node_modules/appium-uiautomator2-server/apks/appium-uiautomator2-server-v1.18.0.apk --override’