What is the url you are using to access? Are you using the host app on plex.tv domain? Or are you using the local hosted app from your server?
If using the hosted app check my reply in this thread: Chrome browser shows connection is not secure?
If you’re not using the hosted app, best case is to use the hosted app to avoid this error. The error you are seeing is that you have loaded the application via http instead of https, or you loaded via https and it could not validate the domain to a certificate.
