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:
||100 posts per month|
||1000 posts per month|
||5000 posts per month|
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.