The schema can be enabled on the [site_reviews] shortcode (which is what you use to display your reviews), and on the [site_reviews_summary] shortcode (which displays a summary of your reviews.
The AggregateRating schema is what Google uses to show the rating snippet in the search results. Both shortcodes will generate the same AggregateRating schema. However, if you use the schema option on the [site_reviews] shortcode, then it will also generate individual Review schema for each visible review on the page.
The reason why the Review schema is not generated on the [site_reviews_summary] shortcode is because that shortcode does not output the reviews on the page, and one of the guidelines for using Schema is that it should represent the page content. If Site Reviews added schema for Review that were not shown on the page, Google would detect that as misleading and possible penalise your page rankings.
To summarise:
1. If you are using both the [site_reviews] and [site_reviews_summary] shortcodes on the page, then use the schema option on the [site_reviews] shortcode to take advantage of the additional schema for each visible review..
2. If you are only using the [site_reviews_summary] shortcode, then you can enable the schema on that shortcode.
3. The AggregateRating schema is identical for both shortcodes.
One other thing:
You generally should only be assigning reviews to either a post ID or to a category. While you can do both, the plugin will perform better if you only do one or the other.
Categories are useful when you need to assign reviews to something that does not have its own post ID or permalink.
Assigning to post IDs have an additional advantage as you can then use the, “Enable Assigned Links” to display a link to the assigned page in your reviews:

OK, thanks for the clarification. Now, I am testing this one one of my pages for Myolift. This is the code I inserted on the page..
[site_reviews assigned_to=”post_id” pagination=ajax count=5 schema=”true”]
Unfortunately, the stars are not showing up in the google search. According to search console the page was indexed 1 day ago, 3 days after I inserted the schema tag.
According to Google, providing schema does not guarantee that snippets will appear. I suggest you use the https://search.google.com/structured-data/testing-tool to verify the schema on your page. You should see a green preview button if there are no errors.