As a pre-paid customer, you need a positive account balance in order to send any messages. Your current balance is displayed in the top right corner of your screen.
If you need to top up your balance and/or you want to enable auto top-up, click on the profile icon in the top right of your screen and select 'Billing and Payments'. On the next page, select the 'Top Up' tab at the top.
Enter the amount you want to add to your balance and click the 'Top Up' button. Notice that the balance amount in the top right of your screen will update accordingly.
Use the toggle to enable auto top-up. Once enabled, you can set the top-up amount, at what point a top-up should be added automatically, and which payment method should be used to pay for these automatic top-ups. Click 'Save'.
You can disable auto top-up again at any time using the same toggle.