Ensure the IDE recognizes that you have the package. It didn't on mine even after downloading 28, so I uninstalled then reinstalled it after realizing it wasn't showing up under File-Project Structure-Modules-App as a choice for SDK.
On top of that, you may want to change your build path to match.
Slightly related, the latest updates seem able to compile when I forced an update all the way to 28 for CompileSDK, and not just up to the new API 26 min requirement from Google Play. This is related to dependencies though, and might not affect yours