Are you actually using Section breaks to define the Sections? If you're expecting the code to work on something you simply call a Section, but isn't defined via Section breaks, you'll get unexpected results.
Furthermore, deleting by Section is problematic since, if you delete a Section, all subsequent Sections have their index # decreased by one. At least your code is doing the right thing by deleting them in reverse order. The following works for me:
Private Sub CommandButton1_Click() With ActiveDocument If .Sections.Count < 15 Then MsgBox "Invalid Action. At least one Section has already been deleted", vbExclamation Exit Sub End If If Me.ComboBox36.Value = vbNullString Then .Sections(15).Range.Delete If Me.ComboBox34.Value = vbNullString Then .Sections(14).Range.Delete If Me.ComboBox32.Value = vbNullString Then .Sections(13).Range.Delete If Me.ComboBox30.Value = vbNullString Then .Sections(12).Range.Delete If Me.ComboBox28.Value = vbNullString Then .Sections(11).Range.Delete If Me.ComboBox26.Value = vbNullString Then .Sections(10).Range.Delete If Me.ComboBox20.Value = vbNullString Then .Sections(9).Range.Delete If Me.ComboBox19.Value = vbNullString Then .Sections(8).Range.Delete If Me.ComboBox18.Value = vbNullString Then .Sections(7).Range.Delete If Me.ComboBox17.Value = vbNullString Then .Sections(6).Range.Delete If Me.ComboBox16.Value = vbNullString Then .Sections(5).Range.Delete If Me.ComboBox14.Value = vbNullString Then .Sections(4).Range.Delete End With End Sub