Переменная базы данных не установлена.
Private Sub Cmdsave_Click()
Dim db As database
Set db = CurrentDb
For i = 0 To LISTQTY.ListCount - 1 Step 1
db.Execute "Updatable tblstock set boxQty = boxQty + " & LISTQTY.Column(1, i) & " where PID=" & Me.LISTQTY.Column(0, i) & ""
Next
End Sub
Или просто используйте CurrentDb
CurrentDb..Execute "Updatable tblstock set boxQty = boxQty + " & LISTQTY.Column(1, i) & " where PID=" & Me.LISTQTY.Column(0, i) & ""