Good question! “Originality” merely applies to the scenario’s content, not the universe/setting. You’re free to set it in a pre-existing universe. For last year’s contest, we had people use the settings for The Expanse and two published Traveller settings.
Unrelated to the contest, but do be careful when writing adventures for other people’s IP (especially if money is involved). Star Wars 5e is constantly in danger of litigation from Lucasfilm/Disney. While I think the actual risk is low, you should still be careful.