You are unable to send because the mailing list you are trying to send it is larger than the number of email credits you have left remaining for your account.
For example, if you have 30 email credits left in your account but you try to send to 40 recipients you will receive this error message. You cannot make a send to 40 recipients with only 30 email credits left for the month.
Email credits can be defined as 1 email credit = 1 email to 1 recipient. With Flat Rate 250 plan you would exhaust all of your email credits by doing the following 250 email credits = 1 email to 250 recipients.
You can check your email credits and the date of your renewal period by checking your team invites and settings page.
If you wish to send to more recipients and do not wish to wait for the renewal period, you will need to upgrade to a larger plan.