[@header]] show up instead of number

Annesoblt You are obviously working in a structured excel table. That's good! The type of formula you mentioned uses structured reference with the @-sign (meaning "take the cell on the same row") followed by the column and with some square brackets. However, based on the column names you provided the formula should look like this:

=[@sales]/[@[order quantity]]

and if you would use a column name like OrderQty, it would look like this:

=[@sales]/[@OrderQty]

 The extra pair of bracket in the first formula is due to the space in the column name.

You don't really have to worry about that as Excel does this automatically when you construct the formula by pointing at the cells.