Unknown Kotlin JVM target: 21" when I update Android Studio to a new version. Such that I updated to the new IDE "Nharwal" and got this issue.

Your Gradle JDK should be set to jbr-17 (JetBrains Runtime 17.0.14).

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Update your Kotlin plugin version in the top-level build.gradle file.

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Make sure that in your gradle.properties file, the property BuildConfig is set to true.

android.defaults.buildfeatures.buildconfig=true