With the Free Profile, you have a limited number of messages. To send messages, you need to have credits available. You can increase them at any time by making purchases of credit packages. With the Premium Profile, the number of messages you can send depends on the number of credits available, which can vary based on the selected subscription. You can increase the number of available credits at any time by purchasing additional packages.
You can check your available credits through the token icon at the top right.