If you're getting the error shown in the screenshot below, there's a high chance that your Email API key is incorrect or has a typo or missing character (for instance, your Sendgrid API key). Your emails are not going to be sent in this case.
FIX: Generate a new API key from your Email API account and replace the existing one in your Coupon Reach email settings.
If you're only getting this error when you open up a specific campaign's properties, you can check out the Email Settings found there.