The error means you have reached the maximum number of posts you can send this month. 

Each plan (except for the Agency plan) has a limit to the number of posts you can send per month:


Starter Plan
100 posts per month
Pro Plan
1000 posts per month
Business Plan
5000 posts per month
Agency Plan
Unlimited


The number of posts you can send will "reset" at the beginning of each month.


You can either upgrade your plan, or wait until the following month for this number to reset and for your posts to start going out again.