Power Query error when combining files

[DashboardNovice]

I have 5 files, each with multiple tabs. I am trying to combine the "Project" tab from each file. I can't attach the files (confidential data). The headers in all files are in the same order and spelled the same way. All have the same casing. What could be causing this error?

 

[GraH - Guido]

Please share your m-code. Maybe your headers look alike. Trailing spaces, lower uppercase is easily overlooked. With the code we can offer guidance to tweak the m-code and so work around the issue.