Hello, I'm looking to purchase a license for a ready-made game engine for Xonix to use in my project. Ideally, I would prefer a engine developed on Unity. I couldn't find any options on the Unity Asset Store. Could you please advise me on where I can buy such a license? Thank you.

Excuse me, but what's Xonix? And what do you mean by an engine developed in Unity? That's already an engine.

Unity is waay overkill for a game like Xonix. You could make a Xonix clone with Raylib or SDL in a few hours, tops; it isn’t a complex game at all. If all you want is Xonix then Unity will do nothing but slow you down and massively bloat your end result.