This message is displayed when you have exceeded your contract maximum active and subscribed subscribers, this is based on the whole number of your contacts, not just the ones you are sending to. For example, you could have a maximum contacts license agreement of 10,000 contacts and have 24,000 active and subscribed contacts in your system. In this scenario, you won't be able to send any communications until either your subscriber list is lowered or you have increased your maximum subscribers.
The best way to check this would be to go to your dashboard and check how many active and subscribed contacts you have.
The first doughnut shows how many contacts you have. The second shows how many are subscribed and the third shows the contact engagement.
If you would like to see your maximum contacts please see your signup form or consult your account manager or customer care team.