[Hummingbird] RSS feed invalid

I am trying to set up Mailchimp to send blog posts using the RSS feed. However it is showing as invalid.
Checking on this website, it shows Hummingbird to be an issue.

https://validator.w3.org/feed/check.cgi?url=https%3A%2F%2Fstartyourrestaurantbusiness.com%2Ffeed%2F