eclipse Error: Could not find or load main class main java.lang.ClassNotFoundException

Check your build.gradle file and update the application block:

application {
    // Define the main class for the application.
  	// insert the entire name of the class
    mainClassName = 'packageName.MainClass'
}

// Run configurations -> Run -> Run configurations
// In the Classpath tab select "Advanced"
// Add where Eclipse usually put the *.class for the projects, which is (usually) "bin".