Hello,

Please read and find the reasons of the HTTPS errors: https://www.wpbeginner.com/plugins/how-to-fix-the-mixed-content-error-in-wordpress-step-by-step/

Please follow the provided suggestions.

Also, You need to investigate the whole site and delete all the demo images with the external links and upload your images to the site.

If your issue will not solve then I need your site WP Admin credentials to check it for you.

Best Regards.