You can only get drill down on formulas such as =SUM(), only on =ADGET(), or =ADCREDIT() etc. My guess is that this is a specification that was converted from a DOS spec, as the cell reference is in column C and the =SUM() formula isn't doing anything (it's not summing anything). Remove the =SUM() from the formula and drilldown will work.

If there are a lot of =SUM()'s to get rid of, you will find it faster to convert all the cell formulas into strings (replace all <=> with <'=)>, then replace all <=SUM(> with <'=>, then replace <))> with <)> (carefully), then replace <'=> with <=>. (Note that in the preceeding sentence, "<>" are used just to separate the replacement strings and are not part of the strings themselves.)

This is one of the reasons we don't recommend using converted DOS specs as a long term conversion strategy to Adagio.