Syntax Errors in VBA

Try replacing the code below with what you have. You were missing Next for each of your For loops. Do and Loop go together, similarly For and Next go together (they form the basis for the loop).

Here's the code, I've cleaned this up as well to make it easier to follow.

Do Until (Range("I4").Value = 0)
    For i = 2 To lLastrow
        If Range("G" & i).Value = 0 Then
            i = i + 1
        ElseIf Range("G" & i).Value < 0 Then
            Do Until (Range("G" & i).Value = 0)
                For j = 0 To i
                    If Range("F" & i - j).Value < 0 Then
                        Range("F" & i - j).Value = Range("F" & i - j).Value + 1
                    Else
                        j = j + 1
                    End If
                    Application.Calculate
                Next j
            Loop
        ElseIf Range("G" & i).Value > 0 Then
            Do Until (Range("G" & i).Value = 0)
                For k = 0 To i
                    If Range("F" & i - k).Value > 0 Then
                        Range("F" & i - k).Value = Range("F" & i - k).Value - 1
                    Else
                        k = k + 1
                    End If
                    Application.Calculate
                Next k
            Loop
        End If
    Next i
    Application.Calculate
Loop

You are missing Next statement for all you Forloops

Try below code

Do Until (Range("I4").Value = 0)

    For i = 2 To lLastrow
      If Range("G" & i).Value = 0 Then
        i = i + 1
      ElseIf Range("G" & i).Value < 0 Then
        Do Until (Range("G" & i).Value = 0)
           For j = 0 To i
             If Range("F" & i - j).Value < 0 Then
                Range("F" & i - j).Value = Range("F" & i - j).Value + 1
             Else
                j = j + 1
             End If
             Application.Calculate
             Next'You missed this
         Loop
       ElseIf Range("G" & i).Value > 0 Then
          Do Until (Range("G" & i).Value = 0)
            For k = 0 To i
              If Range("F" & i - k).Value > 0 Then
                 Range("F" & i - k).Value = Range("F" & i - k).Value - 1
              Else
                k = k + 1
            End If
            Application.Calculate
            Next 'You missed this
          Loop
       End If

        Application.Calculate
    Next 'You missed this
Loop

This is why proper and consistent indentation matters:

Do Until (Range("I4").Value = 0)
|   For i = 2 To lLastrow
|   |   If Range("G" & i).Value = 0 Then
|   |   |   i = i + 1
|   |   ElseIf Range("G" & i).Value < 0 Then
|   |   |   Do Until (Range("G" & i).Value = 0)
|   |   |   |   For j = 0 To i
|   |   |   |   |   If Range("F" & i - j).Value < 0 Then
|   |   |   |   |   |   Range("F" & i - j).Value = Range("F" & i - j).Value + 1
|   |   |   |   |   Else
|   |   |   |   |   |   j = j + 1
|   |   |   |   |   End If
|   |   |   |   |   Application.Calculate
|   |   |   |   Next '<<<<<<< MISSING!!
|   |   |   Loop
|   |   ElseIf Range("G" & i).Value > 0 Then
|   |   |   Do Until (Range("G" & i).Value = 0)
|   |   |   |   For k = 0 To i
|   |   |   |   |   If Range("F" & i - k).Value > 0 Then
|   |   |   |   |   |   Range("F" & i - k).Value = Range("F" & i - k).Value - 1
|   |   |   |   |   Else
|   |   |   |   |   |   k = k + 1
|   |   |   |   |   End If
|   |   |   |   |   Application.Calculate
|   |   |   |   Next '<<<<<<< MISSING!!
|   |   |   Loop
|   |   End If
|   |   Application.Calculate
|   Next '<<<<<<< MISSING!!
Loop

Now, you have 6 levels of nesting here, and quite much duplication. That's a sign you need to refactor and extract a procedure out of the inner nested code, to eliminate redundancies and improve the code's readability and maintainability (one modification should mean one single place to change the code).

If your code works as intended, I'd suggest you bring it (the whole procedure, or even the entire module!) over to Code Review for a cleanup and tips to improve and solidity your code (e.g. you probably don't need to Calculate as often as you do, and unqualified Range calls implicitly refer to the active worksheet - and this might cause unexpected bugs later!).