I newly started using Ubuntu 18.0 LTS this week, switching from Windows, and wanted to install Android Studio and Flutter back. I followed all the steps as said to install A
I had this issue when I installed on Ubuntu. You need to set env variable ANDROID_HOME to the path you installed the android sdk for flutter to work.
So assuming you installed the SDK in your home directory, open a terminal and issue this command
ANDROID_HOME=~/Android-SDK flutter doctor
If that worked, then add the env variable to your profile to make it persistent, by adding
export ANDROID_HOME=~/Android-SDK
to your ~/.profile script, then relogin to Ubuntu.
Obviously if you installed the android sdk in a different directory, you just need to replace ~/ with the directory, so for example if you installed the sdk to /opt, then the command would be ANDROID_HOME=/opt/Android-SDK flutter doctor
Having checked my android sdk + flutter setup, I also ended up adding the following to my path
~/Android-SDK/platform-tools
You can test this before messing with your profile, by the following command in a terminal
PATH=$PATH:~/Android-SDK/platform-tools ANDROID_HOME=~/Android-SDK flutter doctor
And again change ~/Android-SDK to wherever you installed the android SDK. If it works, change your profile accordingly and relogin to Ubuntu.