It's basically the same question as the one asked here:Android Lint Error "Can't find API database; API check not performed"but for Ubuntu, I didn't find how to adapt the answer given to Windows users.
I don't have Eclipse or Android Studio, I just installed gradle and android-sdk. I am trying to follow the tutorial there :https://developer.okta.com/blog/2018/08/10/basic-android-without-an-ide
I get this error after typing "gradle build".