Ionic4: Execution failed for task ':app:processDebugManifest'

I had a similar issue I think, and found out it was because of AndroidX plugins. Here is what worked for me :

ionic cordova plugin add cordova-plugin-androidx
ionic cordova plugin add cordova-plugin-androidx-adapter

Simply adding these plugins removed any build error.

Also, if you're still using the original firebase plugin you should replace it with one of the fixed forks : https://github.com/wizpanda/cordova-plugin-firebase-lib or https://github.com/dpa99c/cordova-plugin-firebase

And cordova clean android then rebuild.

Note that you also need Android SDK 28 for it to work, and I recommend updating Cordova and ionic as well.

After spending number of hours on this issue finally found the solution. This build failure is due to a Major version release of the Google Play Services and Firebase on 17/06/2019

ionic cordova platform rm android
ionic cordova platform add [email protected] 
ionic cordova plugin add cordova-plugin-androidx 
ionic cordova plugin add cordova-plugin-androidx-adapter

Note: This will resolve your issue

Note: If you are using cordova-plugin-firebase and encountering errors, you can use this plugin which is published as cordova-plugin-firebasex and is fixed to resolve issues caused by the new Firebase SDK:

rm -Rf platforms/android
cordova plugin rm cordova-plugin-firebase
rm -Rf plugins/ node_modules/
npm install
cordova plugin add cordova-plugin-firebasex
cordova platform add android

I hope this will help.

cheers!