gradle reads and executes all build.gradle files in all folders of the included modules. As the error shows, it also tries to execute the root build script of :libraries:my_library.
You have to change your settings.gradle and include the library project by setting its ‘projectDir’:
include ':app'
// Give your library project any module name, i.e. ':sdk'
include ':sdk'
// Then set the project path of the library module
project(':sdk').projectDir = new File('libraries/my_library/sdk')
With this settings.gradle you can reference the library project as gradle dependency with:
compile project(':sdk')