RSS Feed not working

Hi,

We’ve had our WordPress site for some time now (over a year) and using MailChimp to send out rss emails. Our site is:

https://www.larkmead-school.com

We have just done an update on our website (changed the theme and some content) and now our rss feed isn’t working. We contacted MailChimp who said the following:

“MailChimp can only pull in content that’s present within the XML of the RSS feed generated by your site. Generally when we see this kind of message it’s because the content is not being added from the blog to the RSS feed itself.”

We’ve checked our feed with feed validator and it comes up with the error below but no idea what it means or how to fix it.

“This feed does not validate.
line 13, column 16: lastBuildDate must be an RFC-822 date-time: [help]
<lastBuildDate></lastBuildDate>”

Can anyone shed some light on how we get this working again.

Thanks ??