go to file>settings>Android sdk>sdk Tools>and update your support repository that fixed my problem

I encountered this problem as well. It was perplexing to me because the SDK manager showed both ConstraintLayout and its solver as installed. What I did to solve it was the following:

1. Open the SDK manager (the Android Studio nav bar button with a downward facing arrow and a little Android head)
2. Click the "SDK Tools" tab
3. Uncheck boxes next to "ConstraintLayout for Android" and "Solver for ConstraintLayout"
4. Click "Apply" and confirm that you want to uninstall those components
5. When uninstallation is complete, recheck the boxes in step 3
6. Click "Apply" and confirm that you want to reinstall the components

The next time you refresh your gradle projects, everything should work just fine!

first add google maven repository in module level gradle file(important part)

repositories {
    maven {
        url 'https://maven.google.com'
    }
}

then add this line in dependencies:

compile 'com.android.support.constraint:constraint-layout:1.0.2'
compile 'com.android.support.constraint:constraint-layout-solver:1.0.2'