If you do not have enough credits to send your message, you have two options:
Upgrade your account
Purchase additional message credits.
To upgrade your account or purchase additional message credits, go to Settings > Plan. If using Subaccounts, you'll need to go into your master account before going to Settings.
Note: only an account Owner or Admin for the master account has access to billing and plans. By default, the email address that created the account is the Owner.