This is the solution for the android studio error message: Emulator: PANIC: Cannot find AVD system path. Please define ANDROID_SDK_ROOT

To solve this, go to tools>AVD and make sure you click on all the required tools by clicking on the download link on the far right of each option.

You may have to reboot. Some users have said, no reboot is necessary. So if all fails, reboot.

Hope that helps.