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: