Your approach and the calculation are correct, the only problem is: the report is filtered by sprint properties “Is Closed” = Yes and “Sprint Story points committed” > 0 . Neither of those values are available for the top row of the calculated member: when you add this member (collapsed) in rows, it is instantly filtered out as not matching.
You may want to remove those filters first, then add the member, expand it and only then set filters back.
Probably, a simpler option than creating a new member would be adding, in columns, a measure “Sprint Start date” and then order by this column; then remove the column, order conditions.