Assuming that you know how to create a hash key, I suggest you to follow the below steps to add a hash key in FaceBook
developer console of your app,
1. After creating a app, click on settings from the left side panel.
2. Select add platform.
3. From the pop up, select Android as your platform.
4. There you will find fields to enter Package Name, Class Name, and hash Keys.
5. Fill up the respective fields.You can enter many hash keys as well.
Note : If you enter wrong hash key, then it will show you the same error again.
hope it helps.