& did not start a character reference

It looks like you must have figured out it was a content issue because the page is validating now.

But for example, if my content says “Aesthetic Design & Photography”, I need to code the ampersand as & amp; (without the space) because the ampersand itself is a reserved character in HTML.

Browsers are pretty good at displaying the page correctly even if you don’t code the ampersand properly, but as you found out the page won’t validate.