There's a specific kind of frustration that builds when you've done the hard work, cleaning your data, removing duplicates, getting everything table-ready, only to hit a wall at the final step. That's exactly where this user finds themselves, and the error message they're staring at isn't just unhelpful; it's misleading. The problem isn't the append operation. The problem is that Power Query is telling them, in its own cryptic way, that something upstream is broken, and it refuses to move forward until they fix it.
Here's what's actually happening: when you remove duplicates in Power Query, you're creating a series of steps that the engine has to replay every time you refresh or modify your query. If any one of those earlier steps hits a snag, a type mismatch, a null value where none should exist, a column that doesn't match between tables, the whole chain stops. The error message they're seeing is Power Query's way of saying, "I can't append these two sheets because I can't even finish building the first one cleanly." It's not about the append at all. It's about the foundation.
The practical takeaway here is simple: don't fight the error message, follow it. Open the Power Query editor, click through the applied steps on each of the two tables, and look for the one with a warning icon. That's where the real issue lives. It could be a column that's been renamed between sheets, a data type that changed after the duplicate removal, or a row that got transformed into something unexpected. Once you resolve that step, the append will likely work without complaint. The user's instinct to check the file format or the table structure is understandable, but it's a distraction. This is a debugging exercise, not a structural one.
What's worth remembering here is that this error isn't a dead end, it's a prompt. It's asking you to understand the sequence of transformations you've already applied, not just the final action. That's the real skill in working with tools like Power Query. Anyone can click through a few menus, but the people who get comfortable with the logic behind the steps are the ones who stop getting stuck. So before you blame the software or assume you need a different approach, take a breath and trace your own steps. The answer is already sitting in the query editor, waiting for you to look at it directly.