Generally long and winding paths down steep hills are not a great idea. Did you export and test in game? Usually as long as that light blue line is intact, sims will be able to route that path.
It could be a steepness issue. Check that box "include steepness". If everything turns red, the elevation is too steep for sims to route.
Those red lines are boundary boxes of objects, so an object might be the culprit and block routing. It might be a badly made CC object or any other object, maybe buried underground. Only removal of that object will help in that case. If you can't find anything there, it might help to clear the global layer, but there is no guarantee.