Package "Android Emulator" with revision at least 28.1.9 not available

Thanks to @JulianC I also set the emulator. I tried to download it standalone from the official site, but couldn't. Then switched to Canary Channel. I pressed Tools > SDK Manager, then Updates label and changed channel. Also pressed Check Now button and dismissed a dialog (Remind Me Later).

Then switch to Android SDK and updated needed emulators.

Then reverted back to Stable Channel.

After several days of work I faced some visual bugs in the emulator. Sometimes I have to restart virtual devices from AVD Manager with Cold Boot Now:

Sometimes I recreate virtual devices in AVD Manager.

Based on https://developer.android.com/preview/release-notes:

Note: If you try to download the Android Q Beta 2 emulator system images from within the emulator, you will be prompted to download the latest version of the emulator. If you are on the stable channel of Android Studio releases, then you might need to switch to the canary channel to be able to download this latest version of the emulator. You do not need to download all of Android Studio from the canary channel—you can download only the emulator from the canary channel from within the SDK Manager.

You have to use the Android Studio Canary build to get an updated emulator. This worked for me

As stated in the other answers, installing the update from the canary channel solves this problem. However, if you don't want to install Android Studio from canary channel, you can just use the sdkmanager tool. Just run this in the terminal:

sdkmanager --update --channel=3