If cell Cx on "Detail report" = "Pr Code" then the loop should look at the the corresponding column "I" on "As Built" associated with that "Pr Code" and add those numbers together. Would it be easier to say something like: 
For x=1 to 100001
If sheets("Detail Report").Range("E" & x) = Sheets("As Built").Range("L" & X) then
      Worksheets("Generalized Report").Range("C16").Value = Worksheets("Generalized Report").Range("C16").Value + WorksheetFunction.Sum(Worksheets("As Built").Range("I" & x).Value)
End If
The value in Column E on Detail Report would match the value in Column L on As built to execute the loop. I have tried it both ways and I still run into an issue that adds multiple times or adds wrong.