Make sure your library
module has its own build.gradle
file inside it. It happens when any of your module inside the project doesn't have its build.gradle file.
Your Project structure should look like this
|+----Your_Project
|+---libraries
| +--library
| +--src
| +--res
| --build.gradle
| --AndroidManifest.xml
|+---app
+--src
+--res
--build.gradle
|---settings.gradle
|---build.gradle