Welcome to the forum.
The rejection reason means that this was rejected by Niantic’s Machine Learning model, which could not detect anything it understood to be eligible about the nomination. I think an unmarked sledding hill is going to fail point 2 of the Acceptance Criteria https://niantic.helpshift.com/hc/en/21-wayfarer/faq/2771-acceptance-criteria/
Must be a permanent physical, tangible, and identifiable place or object, or object that placemarks an area
So this is not one that I would personally keep trying.
Just a note for you to decide aggressively you want to pursue this nomination, if I have found the correct location, it would not be eligible to become a pokestop anyway.
There is a topic that explains this issue here: Why an accepted Wayspot may not be used in Pokemon Go (S2 cells and Pokemon Go)
Lots of folks on the forum are great at looking on Maps and making suggestions of points of interest you may want to submit. I am not the best at it but will have a look.