I've installed Android Studio 3.0, and have compiled and run applications successfully. Device File explorer pane has the above error message in red, and nothing else.

Solution of Android Debug Bridge not found. Simple following 4 Steps : <ol> <li> Open File -> Project Structure -> Project • see if SDK not Selected.. <img src="https://i.stack.imgur.com/JJjfA.png" alt="enter image description here"> </li> <li> Select any latest SDK OR Select Supported version for your system. <img src="https://i.stack.imgur.com/ZmGrb.png" alt="enter image description here"> </li> <li> Then Open File -> Invalid Caches OR Restart Select. <img src="https://i.stack.imgur.com/BJb4m.png" alt="enter image description here"> </li> <li> You can See ADB Error Goes and Emulators Function on the Screen. <img src="https://i.stack.imgur.com/Xm51F.png" alt="enter image description here"> </li> </ol> ☻&hearts; Done. Keep Code.