You could be getting an error message as the fields used for Calculating [ModTempIns] are created after summation. So the place where you are doing this is correct.
Also make sure you are not dropping the table/field. Also please check the Table viewer and look for the field in that table.
Thank you, Ajay.
Adding it at the end of the script does not give an error, but the field is not visible/accessible either. It is not in Table viewer as well.
Sum(CommCorrectedModTemp) as [Daily ModTemp],
Group By Date
[Daily ModTemp]/ [Daily IRR] as [ModTempIns]