Sort Order - what does this meassage mean?

I have selected Type Index and this is the error i get.
In this instance what do i do?

It’s likely that you have made your index column off the column your are trying to sort. - ie. the ‘Type’ column

You need to use the logic you have in your ‘Type’ column to create the index.

So similar to the formula you have but just the 1 & the 2 instead of Revenue and Expenses