NET:ERR_CERT_SYMANTEC_LEGACY

Why this happens

This error comes from using an old certificate that was issued by symantec (tk or one of its partners?)

If you or your customers are seeing this error, this means you’re using Chrome 66 or later and Firefox 60 and your site is using a certificate that was issued by symantec prior to June 1, 2016

Because these certificates were not been generated in a secure way, browser manufacturers began to display these warnings to users

How to fix this

Since this issue is entirely related to the certificate you have and not its configuration, the only option is to reissue the certificate.

To do this, contact your SSL vendor, or use Let’s Encrypt for free, automated SSL certificates.