If you have one criteria, you can just use SUMIF:
=SUMIF(criteria_range, criteria, sum_range)
=SUMIF(AH:AH,"",F:F)
If you have multiple, you can use SUMIFS:
=SUMIFS(sum_range, criteria_range_1, criteria_1, criteria_range_2, criteria_2, etc.)
=SUMIFS(F:F,AH:AH,"",AD:AD,D6)
These are examples, insert your actual ranges you'd like to refer to. The AH:AH,"" bits are checking to see if the cell in that column is blank for the given row(s).