Three possible causes:
On the Free plan. "Upgrade to Pro to send email blasts to your list."
Not yet cleared. "Your first send with imported fans may be reviewed, it clears within 48 hours and you'll get a notification when it's done. New accounts can send after 72 hours."
Out of credits. "You're out of email credits. Buy more in Guest List, Credits. Pro and Lifetime get new monthly credits on the 1st."
Contact support if none of these apply.