Usually when you don't have enough credits to send, even though it might seem like you have enough, you're probably trying to send an enhanced message or you're simply trying to send to too many people.
Enhanced messages costs (2) credits per person to send. These are great because they allow you to send longer text messages, or messages with pictures.
When you're about to send an enhanced message, the whole screen turns blue and there is an alert at the top. Otherwise, you have to keep it below 160 available characters with no image to send as a standard message.