You can check your Email Quota here
From the left panel in your dashboard -> Select Marketing -> Campaigns -> SMS and Emails
If the Email quota becomes Zero you need to purchase additional Emails from the store apps section in order to continue scheduling the Email Campaigns.
If you are on a Free online store plan the default quota will be Zero. If you are on a premium plan a default Quota of Emails is provided only the first time when you use your Email app.