Thanks for providing me that info!
Currently, the latest macOS version is Big Sur 11.6. I would highly recommend updating to this version if possible as OS X El Capitan 10.11.06 was released to end-users in 2015. Google recommends using a recent operating system to ensure you continue to get the latest Chrome versions and features without any issues.
There are a number of reasons why your website’s SSL certificate might be considered invalid by Google Chrome. Some examples include: Errors during the installation process, your SSL certificate has expired, or your SSL certificate is only valid for the main domain and not the subdomains.
When you access your website via Safari, are you met with any SSL errors via your Domain section of your Shopify Admin, and did you purchase the domain through Shopify or a third party?