create a for loop within a for loop

Maybe this :

Sub mil()
    Dim k As Long

    For i = 3 To 15
        If Range("C" & i).Borders(xlEdgeTop).LineStyle = xlLineStyleNone Then
        Else
            For k = i + 1 To 15
                If Range("C" & k).Borders(xlEdgeTop).LineStyle = xlContinuous Then
                    Range("D" & i) = Range("C" & i) / Cells("C" & k)                                   'Divide by the closest cell with a top border
                    Exit For
                End If
            Next k
        End If
    Next
End Sub

Much better when we can see the file. Macro is totally different.
Try this instead :

Sub test()
    Dim rg As Range, c As Range
    
    For Each rg In Columns(1).SpecialCells(2, 1).Areas
        For Each c In rg
        With c
            .Offset(0, 1).Formula = "=" & .Address(0, 0) & "/" & rg.Cells(rg.Rows.Count).Address
        End With
        Next c
    Next rg
    
End Sub

It finds all areas separated by a blank row and the adds the formula.
I've added some comments in the code.
If you need more help, try using the F1 key (help) on the keywords such as .Address, .SpecialCells, etc...

Sub test()
    Dim rg As Range, c As Range
    
    ' Columns(1).SpecialCells(2, 1).Areas
    ' In column 1, find all areas that contain cells that are constants (2) and numbers (1) only
    ' Loop through each area (rg) of the areas found
    For Each rg In Columns(1).SpecialCells(2, 1).Areas
        'for each cell (c) in the range area (rg)
        For Each c In rg
            With c
                'offset 1 column formula = adress of the cells (rows and columns relative (0) and divide by
                '                          the last cell in the area (rg.cells(rg.rows.count) (rows and columns absolute (1 per default))
                .Offset(0, 1).Formula = "=" & .Address(0, 0) & "/" & rg.Cells(rg.Rows.Count).Address
            End With
        Next c
    Next rg
    
End Sub

Here's a version with one loop.

    Dim rg As Range
    
    For Each rg In Columns(1).SpecialCells(2, 1).Areas
        rg.Offset(0, 1).FormulaR1C1 = "=RC[-1]/R" & rg.Cells(rg.Rows.Count).Row & "C[-1]"
    Next rg

Which code are you referring to and how did it not work?