The reason this won’t work for you is because Slider::Listener has a pure virtual function you need to declare in order to use the class.
If you see the documentation here, you’ll see that the function sliderValueChanged() is virtual function. Be sure to declare the function with the keyword override as well in the header file.
I have a video from a while back that shows how to do this, but I would now advise you to use the audioProcessorValueTreeState instead of Slider::Listener. This will allow you to seamlessly replay value changes from your slider to your processor without needing to inherit Slider::Listener. The video shows you both ways but you should use either one or the other.
There have also been recent updates to the audioProcessorValueTreeState which I’ve outlined here.
I know it’s a lot to get through but let us know if you have any questions.
Hope this helps!