That is an error message associated with your SSL certificate.
This can happen if you’re linking customers to a part of your website that isn’t on Shopify.
It can also happen if your site doesn’t have an SSL certificate activated. That doesn’t seem to be the case though, as when I go to your website, I do see an SSL certificate issued.
One thing worth noting though is that your DNS records for the ‘www’ version of your domain, aren’t set correctly.
See here: https://who.is/dns/roseelladesign.com.au
Your ‘www’ version of the site has an a-record (which shouldn’t be there) – you should only have the single a-record for the main domain (without the ‘www’). Also, your CNAME record for ‘www’ should be changed to “shops.myshopify.com”.
If you make the above changes, it should hopefully resolve the issue with your customers seeing issues with the SSL certificate. If customers keep reporting that issue, after making the above changes to your DNS records, then you’d want to reach out to Shopify’s support team directly, for further help troubleshooting.