I had trouble installing Norton Security (& Family) on a new LG G5 with a fresh Android system update.
Installation of the apps went OK, but a few seconds after launching a popup appeared with error message "Unable to sign in -- Certificate can't be validated on your device. Please make sure your device date is correct." The device was connected to a cellular network, so date/time were fine. Rebooting didn't help.
I contacted Norton support via chat, and was advised that "Android System WebView" had to be enabled. There was a button to do so both in the Play Store listing and the App configuration dialog, but neither was effective.
It turned out that, as of Nougat 7.0, Google Chrome is the default handler for 3rd party applications with a web interface, replacing the now deprecated WebView. WebView can be re-enabled by (temporarily) disabling Google Chrome from the app list dialog, then enabling WebView and updating to the latest version. This isn't a good long-term solution because it naturally disables the entire Chrome browser. Fortunately, it sufficed to configure Norton Security, after which I re-enabled Google Chrome. NS seems to work OK with this configuration.
I had to repeat the process (temporarily disable Chrome, enable and re-update WebView) to install & configure Norton Family.
Hopefully this information is useful to other bleeding edge Android users.
Norton should address this issue (hence the need for a solution) in the future by removing the dependency on the now-obsolete Android System WebView app.
I'm hoping there won't be any longer term issues (e.g., when there is a Norton app update).