Make sure you have enough Balance in your account to buy the Add-on (or it will fail). You can do a one-time top-up for the amount of the Add-on.
Bonus for Add-ons is that many of them don't "expire" but roll-over until used up. In other words, if you buy minutes/data, every cycle, Public Mobile will deduct from your monthly allowance first before going into your Add-on. 🙂