Why can't I add my observation to this project?

If you're unable to add an observation to a project, it is likely a Collection project, which is essentially just a filtered search on all observations. Observations are never "in" a collection project; they either meet the project’s requirements and are automatically displayed when the project page is loaded, or they don’t. If you navigate to a collection project's page on our website, you can click on "View Yours" to see which of your observations meet the project's requirements.

You cannot add or delete individual observations from a collection project. However, note that you can always edit the project to tweak its settings. Because a collection project is a just saved observations search, there is no way for you to exclude specific observations from appearing on a collection project’s page. It's only possible to manually add or remove observations from Traditional projects. Please see the Projects on iNaturalist page for more information.