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VirtualButtonAbstractBehaviour not found

Hello everyone, I started a project in AR and until now everything worked fine, but now I want to implement a virtual button that has the following method in the script:

public void OnButtonPressed (VirtualButtonBehavior vb) {      Debug.log ("works!"); }

however, the message doesn't activate, I have seen many posts and it is a recurring problem, the posts mention that the method should be changed to this:

public void OnButtonPressed (VirtualButtonAbstractBehavior vb) {      Debug.log ("works!"); }

However, when adding the word "Abastract" the following error appears:

"The type or namespace name 'IVirtualButtonEventHandler' could not be found (are you missing a using directive or an assembly reference?) "

I have the Vuforia dependency added to the beginning:

using Vuforia;

My version of Vuforia is 8.3.8 and the version of Unity is 2019.2.2f1

Any idea that what can be?

Regards!