Invalid Gradle JDK configuration found?

I am very new in Android Studio. I have a problem with gradle. it says,

Error:Invalid Gradle JDK configuration found. Platform SDK does not point to valid `JDK` `(C:/Program Files/Java/jdk1.8.0_20)`. Consult IDE log for more details (Help | Show Log)

then what should I do to solve that?

What is gradle Android?

Android Studio uses Gradle, an advanced build toolkit, to automate and manage the build process, while allowing you to define flexible custom build configurations. Each build configuration can define its own set of code and resources, while reusing the parts common to all versions of your app.

I got this issue in Android studio 4.2 canary, and I had to delete those generated files: