When you follow Liferay tutorials, especially when creating a portlet, this error usually appears.
This just means that the tools.jar is not found under the directory configured as Java home.
This is solved right clicking the Gradle Task "deploy", and selecting "Open Gradle Run Configuration...".
Go to the Java Home Tab, and set the correct directory to the JDK directory
Click Apply. Try to run again the "deploy" task. It should run fine now.