Lightroom mobile is not yet Fully supported on Chromebook as it requires either iOS or Android OS (not ChromeOS) to run.
Many apps including Lr Mobile and Photoshop are still being tested for compatibility,so in near future they might work properly.
Has there been any improvement on Lightroom compatibility with ChromeOS?
I have the same problem with the android app. But the web app does see the raw files and will import them. The web app at this time appears to have much less functionality than the android app. But you can always do the import using the web app and then go to the android app to do editing.