NoClassDefFoundError: org/jetbrains/plugins/gradle/tooling/ModelBuilderService

It's a known issue caused by the incompatible Kotlin plug-in update. The issue is already fixed. The solution is to either update Kotlin plug-in again to get the version compatible with IntelliJ IDEA 2020.3 release or update IntelliJ IDEA to 2020.3.1 version.

In my case I just needed to disable kotlin. if you dont need kotlin at the moment you can disbale it in idea. just got to Plugins>Installed->Kotlin then disable for all projects.