You cannot say 'on a whole'. You can say 'as a whole' which means altogether or all aspects considered. Or you can use 'on the whole' which means considering everything.

The phrase is normally "On the whole". It means "overall" or "in total". The author is saying that even though some government and business have done well, most have not, and so the average is very poor performance.

on a whole = collectively

on a whole = generally