I had this exact same problem, and it was super frustrating. I started building a site two years ago and abandoned it. Recently picked it back up, had to update WordPress, WooCommerce, my theme, plugins, etc, and then all of the problems started. I was getting the same error message in WooCommerce, but my hosting provider and every SSL check website was showing me as secure. Here are the things I tried that did not resolve the issue:
1. Changed both URLS to include HTTPS
2. Switched PHP from 7.0 to 7.2 because I was getting a potential warning message regarding that
3. Re-uploaded a lot of my images because I was occasionally getting an SSL error message for random ones on SSL check sites like my logo and my shopping cart icon.
4. Siteground did the following trying to fix it: It was a mixed content of the website – I have added a directive to your .htaccess and this has fixed the mix content error
4. Other minor tweaks here and there, nothing worked
What finally worked was simply deactivating the WooCommerce plugin, reactivating, and clearing cache, the error message went away immediately after that. I felt so dumb to have spent 8+ hours trying to figure this out and never thought to deactivate the WooCommerce plugin although I did try activating and deleting other plugins that I thought may have been giving me issues.