When adding a new payment method, you have to authorize Clickatell to store the card for future transactions, and also indicate that you would like to enable the card for automatic payments in future.

If you are currently using the old user portal:

Automatic system payments is a global account function which must be set to 'on' before Clickatell can process any automatic payments for subscription services such as long numbers and short codes. This setting can be configured on the Payment methods page when adding a default payment method. Once configured, we’ll use your default payment methods for all subscription service renewals and you’ll be able to enable auto renewal for individual subscriptions.