android manifest merger failed, gms play services / firebase

I solved the problem by adding:

    configurations.all {
    resolutionStrategy.eachDependency { DependencyResolveDetails details ->
        def requested = details.requested
        if (requested.group == 'com.android.support') {
            if (!requested.name.startsWith("multidex")) {
                details.useVersion '26.0.0'
            }
        }
    }
}

from here.

The tooltip recommended adding tools:replace="android:value"' to meta-data but this throws another error, so im going with the solution above

It's happening because two versions of support libraries are clashing. On top, you have declared

buildToolsVersion "26.0.1"

and in dependencies, the version is 26.0.0

compile 'com.android.support:design:26.0.0'

Just change the support library version to 26.0.1 and it will work fine. I did the same, worked flawlessly in my case.

add this line at the end of app level gradle file

apply plugin: 'com.google.gms.google-services'

I solved it by adding this in AndroidManifest.xml within the <application> tag at the very bottom:

<meta-data 
  tools:node="replace"
  android:name="android.support.VERSION"
  android:value="26.1.0"  // <- The max version you see in the error message. For me it was 26.1.0
/>

Then add these two attributes to the <manifest ... > tag:

xmlns:tools="http://schemas.android.com/tools"
tools:node="replace"