If you are left with fewer credits in your account than the number of subscribers to whom you want to send an email marketing campaign, you will not be able to dispatch your email campaign. To solve this, you can buy more credits and move forward with your campaign dispatch.
When using credits, one credit equals one email sent to one recipient. If you want to send a campaign to an email list of 60 recipients and you only have 59 credits, you need at least 1 more credit before you can dispatch your newsletter.
It is a good idea to always check your remaining credits before sending out a campaign.