Skill studio is the legacy UI which you no longer have access to if you are working with the agent builder as I see in other threads. This is explained in the root page of the section you are referring to. This section of the doc will be retired once all our customers have moved over.
You shall use the new / equivalent tool & flow features in the agent builder which I see you already are looking at.
Note that the assistant builder remains available as-is, just without the skill-based actions but we encourage new customers / projects to focus on agents which are the strategic technology moving forward.
I hope this makes sense, thanks!