Those are no normal buttons, but embedded objects. They have no individual timeline, you cannot group them.
Maybe explain what you want to do exactly? And please tell which version you are using.
I have an interaction slide, with 4 types of interactions on it. The 4th one, is a D&D Interaction, which user has to answer to advance in course. Problem is, the D&D interaction should only appear when the user complete the others. I was able to hide all the draggable objects and drag area, but not the Submit and Undo buttons. They are showing when the user enters this slide, they should be hidden until they click on a button.
Edit: I'm using the latest version. 220.127.116.116
That simply will not work, because all Drag sources and Drop targets have to be on the slide timeline from the first frame.
You will need to have the D&D slide separately. Transition to that slide from the previous slide can be seamless, learner shouldn't even be aware there are two different slides. You can have the objects of the previous slide timed for the rest of the project, so that they show up on the D&D slide and hide them when you enter the slide after the D&D slide.