PowerBI Error Query.

Did you get no errors in the steps preparing the query but then the errors show when 'Close and Apply' is pressed?

 

Many times these errors are either in data in a column not being the right type assigned in Power Query or sometimes in Excel files, there are additional tables or data in there which breaks the steps you create in Preview.

Powerbi is pretty good at showing the data which throws errors (as in the picture above)

To debug : I suggest looking at the steps in the Transform data and removing as many as possible as an experiment (to keep it simple) and import everything as text columns.  If it succeeds, add in the query steps you want one at a time and see what breaks.