I have an app that should be tested on an Android emulator. After setting to the React Native and Android environment (Including the environment variable). I can not run my app on the Android emulator.
Message spoof: 
I'm using windows 10 x68 with JDK, JRE 1.8 installed and configured (Environment Variable)
The displayed error is related to the location of the JRE, Gradle can not find the tools.jar file.
This file is usually found in the lib folder inside the JDK folder, notice that your variable is pointing to the JRE folder.
Modify the JRE path to the JDK folder.
You might find it at:
c:\Arquivos de Programa\Java\jdk1.6.0_26\lib\tools.jar