Android studio Namespace 'tools' is not bound！！

For example

<activity
    android:name="com.google.android.gms.ads.AdActivity"
    android:theme="@android:style/Theme.NoTitleBar.Fullscreen"
    tools:replace="android:theme"
    />

Add

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

at the top of the manifest (or activity).

<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.mypackage"
    xmlns:tools="http://schemas.android.com/tools">