Update April 2021: Google has announced that they are discontinuing email support on FeedBurner. This feature will no longer be available from July 2021.
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 https://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: