I am having the exact same problem.
Site was working fine for at least 4 months when I migrated from a legacy shopping cart program on a sever, to Shopify.
Using the common computer browsers, people get this error and can't see the page.
Using phone browsers or streamlined computer browsers, they can get to the site "unsecured" but then get the warning at the checkout page.
Inspecting the Chrome error shows "SSL Certificate Invalid".
The warning page says "NET::ERR_CERT_COMMON_NAME_INVALID" but I don't know if this is because of the SSL cert that was on the previous host or the Shopify host.
Do I need to revoke the previous SSL certificate from Godaddy, where the site used to be hosted? I don't want to do that without clarification, as I said the setup was working fine for the last 4 months. The old SSL cert is valid through end of 2021. Obviously the Shopify SSL cert should "never expire".