I'll tell you what I think I see, but I may be mistaken so don't hesitate to
straighten me out if I'm misunderstanding something:

1) You declare Last_Cell as a Range, but I don't see any code that tells
your program what cells it refers to. If you defined that elsewhere in your
program, like this:

Dim First_Cell, Last_Cell as Range
..
Set Last_Cell = <some range
..
For Each First_Cell In Last_Cell
If Not Last_Cell Is Nothing Then Exit For
First_Cell.Formula = "=dsum(H15:M2000,m15,ak5:al6)"
Next First_Cell

....that's fine; but it literally is what you put below then I think what
happens is that there are no cells in Last_Cell and the loop executes with
zero iterations.

2) If, as you say, there are definitely empty cells in Last_Cell, here's the
next problem (maybe): Your code specifies that the first non-empty cell in
the range will cause it to leave the loop. So if you meant it to fill in
this formula for every empty cell in the range, what it'll really do is fill
in the formula only for the empty cells that come first in the range, and
none of the empty cells that come after a non-empty cell. And if the first
cell in the range is not empty, then it won't fill in the formula anywhere at
all. See what I mean, here? If that's the problem, you should modify the
loop to look like this:

For Each First_Cell In Last_Cell
If Last_Cell Is Nothing Then First_Cell.Formula = _
"=dsum(H15:M2000,m15,ak5:al6)"
Next First_Cell

I don't see any other problems. My modifications are just because I think
the above is a little easier to interpret, not because your If statement had
an error in it. (But it does seem silly to activate First_Cell and then use
ActiveCell.Formula when you could more easily have used First_Cell.Formula in
the first place.)

--- "Jayne22" wrote:
I'm trying to fill in a formula for each empty cell in Last_Cell (a range
that i declared); if the cell is not empty, it can skip that cell. Why
doesn't this work when there are definitely empty cells in the range?

Dim First_Cell, Last_Cell as Range
For Each First_Cell In Last_Cell
If Last_Cell Is Nothing Then
First_Cell.Activate
ActiveCell.Formula = "=dsum(H15:M2000,m15,ak5:al6)"
Else
Exit For
End If
Next First_Cell

End Sub