Help! First AMP warning - what did i do wrong and how can i fix it?

Site URL: http://www.richardrobertsarchive.org.uk

As you can probably tell from the title I'm not a web developer. I signed up with SquareSpace to avoid having to do things like this.

Ive taken a look at the more information page for the warning on Google but it might as well be written in Swahili.

Is this something I need to be worried about? Its appeared after posting several new blog posts so an educated guess says theres something wrong in the posts.

If anyone has a link to maybe a blog post that explains this and the solution very simply for people like me who only know basic formatting html that would be very helpful too.

Thanks!

What is the warning and what does the more information page say? It's going to be hard to help without more detail.