You are using

implementation 'com.google.firebase:firebase-messaging:19.0.1' 

Firebase migrated to AndroidX in the latest release. It means that you are using both, support libraries and androidx libraries.

You can:

• migrate to androidx as described below
• downgrade your firebase dependencies (but it is not a real solution because you have to migrate before or after)

You can check the official release notes:

Warning: This release is a MAJOR version update and breaking change. The latest update to Google Play services and Firebase includes the following changes:

Migration from Android Support Libraries to Jetpack (AndroidX) Libraries. Libraries will not work unless you make the following changes in your app:

• Upgrade com.android.tools.build:gradle to v3.2.1 or later.
• Upgrade compileSdkVersion to 28 or later.
• Update your app to use Jetpack (AndroidX); follow the instructions in Migrating to AndroidX.

You are using

implementation 'com.google.firebase:firebase-messaging:19.0.1' 

Firebase migrated to AndroidX in the latest release. It means that you are using both, support libraries and androidx libraries.

You can:

• migrate to androidx as described below
• downgrade your firebase dependencies (but it is not a real solution because you have to migrate sooner or later)

You can check the official release notes:

Warning: This release is a MAJOR version update and breaking change. The latest update to Google Play services and Firebase includes the following changes:

Migration from Android Support Libraries to Jetpack (AndroidX) Libraries. Libraries will not work unless you make the following changes in your app:

• Upgrade com.android.tools.build:gradle to v3.2.1 or later.
• Upgrade compileSdkVersion to 28 or later.
• Update your app to use Jetpack (AndroidX); follow the instructions in Migrating to AndroidX.