The feature is added in the latest version of theme so please update your theme from file bellow. There will be a new option under Theme Settings > Portfolio. It is possible that old portfolio item commnets will need to be activated manually by editing its pages.
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You can update it by either deleting existing theme and then install latest theme version of it or use the following plugin to update the theme. https://wordpress.org/plugins/easy-theme-and-plugin-upgrades/
After activating the above plugin just install the downloaded theme on your site.
You won’t lose your site content after updating the theme but for safer side you can take backup of your site before updating the theme.
There is a bug in this beta, though. A comment section is also added in the gallery layouts (grid showing all of the images). Comments should probably only be enabled pr. image (after clicking on them, like with blog posts). Or else the home page will get this at the end: (see screen grab attached) when adding a portfolio gallery.