To remove this message, you have to configure HTTPS on your website.
To configure HTTPS on your website, you have to add (or modify) the following parameters in the DNS settings (not to be confused with NS-servers) on your registrar's control panel:
You should add only one A Record with an "@" value and only one A Record for the "www" subdomain.
After that, wait up to 24 hours for the DNS records to update (the exact amount of time depends on the registrar). Then go to Site settings → SEO → HTTPS settings. You will see that an option to manage HTTPS is now available. Click Manage and switch on HTTPS.
Learn more on how to configure HTTPS in our step-by-step guide: https://help.tilda.ws/https