According to relevant the Android Docs, you need to manually initialize dependency configuration methods that combine a product flavor and a build type (implementation and debugImplementation are examples of dependency configuration methods, as is brieDebugImplementation):

//.gradle.kts eaxmple
val brieDebugImplementation by configurations.creating

dependencies {
    brieDebugImplementation("com.squareup.okhttp3:okhttp:4.12.0")
}

//.gradle example
configurations {
    brieDebugImplementation {}
}

dependencies {
    brieDebugImplementation 'com.squareup.okhttp3:okhttp:4.12.0'
}

Found here: https://developer.android.com/build/dependencies#configure_dependencies_for_a_specific_build_variant