Failled GTMetrix test "Specify a cache validator"

I’ve been trying to improve my site’s load speed and getting a failing score for Specify a Cache Validator. I asked Dreamhost support about it and one of their techs looked into my site and reported the following.

This from Dreamhost support –
“As for the GTMetrix result, this would be solved by finding out which
plugin or plugins are causing an issue with our server-side cache, it can
even be the theme. The first step towards that is disabling WP rocket. I
ran a debug and it seems the theme is also causing problems, as it runs
PHP sessions in each call, and it’s quite possibly the one thing setting
the ‘Age’ header to an improper time. If you can swap the theme out and
run a cache test, which you have access to now in the site’s backend
under the ‘Proxy cache’ section, that would let you know if it was the
theme or not.”

I ran the check with the proxy cache they set up and it is reporting “no known cache service is detected on your site”. Also under Age Headers – “Your domain does not report an Age header, making it impossible to determine if the page is actually serving from cache.”

I switched to a different theme and those tests were good.