this thread probably should be merged with this one: https://forum.inaturalist.org/t/how-can-i-allow-people-to-add-observation-in-my-project/32171, since i think it’s just another way of asking for the same thing.
the bottom line is that you can’t manually add observations to collection projects and umbrella projects.
however, if you have a traditional project, you can add observations to the project. see https://www.inaturalist.org/pages/managing-projects and https://forum.inaturalist.org/t/adding-observations-to-a-traditional-project-wiki/13190.