Summary
The “therefore” in Romans 2:1 is pointing back to the entire preceding passage in chapter 1 (Romans 1:18-32)—the fact of divine justice being perfect, absolute, and imminent for anyone who trifles with it. Chapter divisions were added later (are not part of the inspired text itself), so the air of separation that may be introduced by the chapter break is not actually inherent to the text.
Content
We might phrase the general question something like:
What’s the sense of the “therefore” (διό) in Romans 2:1? What exactly is it from chapter 1 that makes it so that they have no excuse? What’s the specific antecedent? Is it just natural revelation generally?
Part of the difficulty is that the chapters are split where they are. Chapter and verse divisions are not part of the inspired text, but were added much later. In this particular case, the passage that we are looking at is still fundamentally tied to everything Paul was saying in the first chapter of Romans, but the chapter break sort of introduces an air of separation that wouldn’t otherwise be there.
So the short answer to this question is all of chapter 1 from verse 18 onward. That is to say, the fact of divine justice being perfect, absolute, and imminent for anyone who trifles with it.