First check that you are logged in. Do this by going to the Settings >> Subscriptions menu option.
On the "My Subscriptions" list of your Account page you should see the service that you are trying to subscribe to. The reason for the problem is that you ordered the service but cancelled the transaction, or didn't complete the transaction. This also occurs when a payment fails and the subscription is placed 'on-hold'.
To reactivate your subscription, just click on the "Pay" option to the right side of the screen.