You must ensure your wallet has sufficient funds to cover the cost of the subscription. You can add funds by redeeming a PlayStation Gift Card or using a credit card/PayPal to add funds directly, as described in the Startselect guide.

Reduces the need to store sensitive financial data on your console Startselect. Things to Consider

If your wallet balance is less than the total cost of the PS Plus plan, you will be prompted to add more funds or pay the difference using another payment method Startselect.