Column1 Not Found" Error in Power Query Folder Combine After Promoting Headers

Hi Waterfront,

It's important you verified all files have an identical structure with matching column names in row 4 of the sheet. And you should move the Removed Other Columns - step to this query (if applicable) and address it in the Transform Sample File as it's logic is applied to all files in the folder. 

If you can't move that step, examine the Combined File query. The process is the same, step through the code by selecting each step one by one from the Aplied Steps, to determine where 'Column1' makes an appearance - once you've identified where and how it comes into being, you can resolve it. 

As for how to extract "1" from "File_1" just use the User Interface, go to: Transform > Extract
There you'll find options to extract the last characters or text after a delimiter - use what suits your data best.

I hope this is helpful