Confused about namespace issue.

I have a grabbable object, and I want it to triger a function of another class ScreenShotManager when it is grabbed, so I announce a variable like this

public ScreenShotManager shotManager;

But it throws an error when compiling:  Assets\Oculus\Interaction\Runtime\Scripts\Interaction\Grabbable\Grabbable.cs(47,16): error CS0246: The type or namespace name 'ScreenShotManager' could not be found (are you missing a using directive or an assembly reference?)

The ScreenShotManager.cs is under Assets\Tests\Scripts  and it doesn't belong to any namespace. 

How could I solve this?