How do I add a header (e.g. "Sales") in the yellow highlighted area as seen in the screenshot below? I have other worksheets on a dashboard that have a dimension in the "Rows" area and a header is added that I can edit, so it looks funny to have this worksheet with no header in the same location.
"Show Field Labels for Rows" is checked and greyed out under Analysis>Table Layout.
You have measure names on rows currently - so it's displaying a name for each measure on each row - but there's no default Measure Names row title for you to edit. The only way to get it would be to pivot your data so that you have one column with each measure name (which you could rename "Sales") and a second column with each value for that measure.