The same issue, but...
I have the same issue (using a Fenix 3 HR), but I made a mistake choosing my last activity. I selected Trail Raunning instead of Running and... Surprise! The Sync works.
I created another activity mode but based in Running and again does not work.
I don't understand...
I know this is an old post, but it’s the top hit in google and I was still running into this issue with my trusty forerunner 235.
I tried “log out NRC uninstall NRC and then restart” fix and it didn’t work.
I tried a bunch of other random stuff from around the internet and non of it worked.
What fixed it for me was me being a dummy.
I realised that when I hit the ‘run’ button on my watch it was categorising my excercise as ‘other’ rather than ‘outdoor run’ or just ‘run’. As soon as I made this change (by hitting the up/down arrows in the watch when I’m the activity) Garmin started syncing with NRC.
Hope my stupidity helps someone somewhere !