The SEO problems, Blog and Speed checkers in Plug in SEO Free all work on any public website. Some of the advice is Shopify specific, but they will return actionable results on other platforms.
The Plug in SEO Plus upgrade is only available for Shopify at this time.
Are you on Bigcommerce, Magento, WooCommerce or something else? Let us know and we'll drop you an email when we launch there.