The latest version being Android 8.1?
Android 8.x makes the notification LED a bit trickier to configure. It gives you a bit more control, but certainly isn't any easier to use. Additionally, you may only be able to configure this option for apps which support Oreo's
Notification Channels feature.
First, check under
Settings > Apps & notifications > Notifications to make sure the
Blink light option is enabled. That's the global setting; without it, nothing will work.
Then
for each app you'd like to use the notification LED, go to
Settings > Apps & notifications > See all apps > [App Name] > App notifications and select the notification category you'd like to adjust. You'll need to make sure that the
Importance is set to
High or
Urgent; you should then be able to enable the per-app
Blink light option.
It's tedious and kind of frustrating (particularly if your favorite apps don't yet support the Notification Channels API), so feel free to give your feedback on this change via
Settings > System > About phone > Send feedback about this device. You may also want to reach out to the developers of any apps which don't support this change - you can find their contact email address on the app's Play Store listing.
Hope that helps!