Check-box should not be marked/unmarked when the sheet is protected

I am trying to make a macro that adds a new checkbox when new data is inputed, but it should only be marked or unmarked when the sheet is unprotected. I tried to do this with the following code:

Range("Z4").Select
ActiveCell.FormulaR1C1 = "=LOOKUP(2,1/(C[-15]<>""""),ROW(C[-15]))"
i = Range("Z4").Value + 1

    Cells(i, 19).Select
    ActiveSheet.CheckBoxes.Add(Cells(i, 19).Left, Cells(i, 19).Top, 60.75, 15).Select
    With Selection
            .Value = xlOff
            .LinkedCell = Cells(i, 30)
            .Display3DShading = False
    End With
    Selection.Characters.Text = "Pagado"

Where the linked cell is locked and therefore should not be able to be changed when locked.

Otherwise, the rest of the code works perfectly.