The Value expression for the text box specifies a scope that is not valid for a nested aggregate.

You are jumping a level of aggregation, that's why SSRS return the message error.

Try:

=Sum(Code.Divide(sum(Fields!WeeklyUnits.Value),Sum(sum(Fields!EstUnits.Value)))
*
Sum((Code.Divide(sum(Fields!EstHours.Value)),Sum(sum(Fields!EstHours.Value,"Job")))))

It should work. I always suggest to carry data directly from SQL for this kind of operation. Just one row more in your table with the sum aggregation. In your case you could have taken the first value of the new column without climbing any level..