Some routers require you to fill out unique IPv6 addresses for both primary and secondary DNS. If you're encountering this error, we recommend filling in this address for secondary:
2606:4700:4700::1113
This is a generic adult and malicious content-blocking domain provided by Cloudflare. It will block most adult websites and security threats.
However, if your router switches to secondary addresses (usually because of a temporary outage), your custom Content Policy rules will not be used on your home network. We recommend only using this as your secondary DNS provider.
There are alternative versions of the IP address (including IPv4 addresses) in this blog post.