What happened with Ubuntu 20.04 at this point:

$ flutter doctor Downloading package sky_engine... 839ms Downloading flutter_patched_sdk tools... 1,504ms Downloading flutter_patched_sdk_product tools... 1,469ms Downloading linux-x64 tools... 9.9s Downloading linux-x64/font-subset tools... 666ms Doctor summary (to see all details, run flutter doctor -v): [✓] Flutter (Channel dev, 2.6.0-11.0.pre, on Ubuntu 20.04.3 LTS 5.4.0-81-generic, locale en_GB.UTF-8) [!] Android toolchain - develop for Android devices (Android SDK version 31.0.0) ✗ cmdline-tools component is missing Run path/to/sdkmanager --install "cmdline-tools;latest" See https://developer.android.com/studio/command-line for more details. ✗ Android license status unknown. Run flutter doctor --android-licenses to accept the SDK licenses. See https://flutter.dev/docs/get-started/install/linux#android-setup for more details. [✓] Chrome - develop for the web [✓] Linux toolchain - develop for Linux desktop [✓] Android Studio (version 2020.3) [✓] Android Studio [✓] Connected device (2 available)

The solution for me was to go to Android Studio, fins SDK Manager whuch was under oprions in the right hand pane and install COmmand Line Tools from SDK Manager. I was then able to fix the licences issue too.