What does this mean "Class 'androidx.compose.ui.Modifier' is compiled by a new Kotlin compiler backend and cannot be loaded by the old compiler"? I am already using the latest kotlin compiler 1.4.0-rc. I don't understand.

Probably you're missing the 

-Xallow-jvm-ir-dependencies

 as per the "declaring dependencies" section of the release notes: https://developer.android.com/jetpack/androidx/releases/compose#declaring_dependencies

I was doing that in project-level build.gradle 😑🙈