If you see the FeedBurner error message above, this means that you need to enable the subscription by email option in your FeedBurner account settings.
How to fix it:
1. Login at http://feedburner.google.com/ and select your feed.
2. Go to the "Publicize" tab:
3. Open the "Email Subscriptions" section on the left.
Make sure that "Email Subscriptions" are enabled.
The video below shows an example of how to enable email subscriptions: