A column that should hold numbers often arrives with a few values that aren't numbers: "n/a", "not rated", a dash. Power Query then guesses the whole column is text. When you change its type to a number, those cells become Error values (DataFormat.Error), shown in red, and the column quality bar shows a red slice.
Errors stop a load and break sums, so deal with them on purpose:
- Transform > Replace Errors swaps every error in the selected columns for a value, usually null (missing) or 0. Null is safer: an average then ignores the row instead of counting a fake zero.
- Home > Remove Rows > Remove Errors drops the rows instead.
- Or fix the text first with Replace Values before you change the type.
Select an error cell to read its message at the bottom of the preview.