seems like the triggering problem if you try to create projected noted in Step 2 of the original post is that “12/31/1999” is an invalid date format. (1999-12-31" would be the expected format.)
it seems like there’s no validation on the screen itself to make sure the date is valid (which is probably a bug that should be fixed), and it fails when the server tries to process the date. so the project gets partially created (the project slug gets registered), but it has no rules. so it seems to get stuck in a limbo state, where you can sort of view the partially-created project from https://www.inaturalist.org/projects/before-2000?collection_preview=true, but you might not be able to edit it from https://www.inaturalist.org/projects/before-2000/edit.
so the original project in limbo needs to be deleted first. if the edit page is not working from https://www.inaturalist.org/projects/before-2000/edit, then it’s possible to delete the project using a call to the API. in Windows, one way to accomplish that would be:
that should display something like {"api_token":"J.W.T"}, where J.W.T will be some encoded string. copy the J.W.T value.
open your Windows start menu, and run “cmd” to open the command line editor.
at the command prompt, run curl -X DELETE "https://api.inaturalist.org/v1/projects/before-2000" -H "Content-Type: application/json" -H "Accept: application/json" -H "Authorization: %JWT%", making sure you replace %JWT% with the value you copied from step #2 above.
…
one more thought: it might be useful to update the title of the thread to reflect the fundamental issue, as opposed to the case where the issue was encountered.
As a curiosity, the oldest verifiable observation at the moment is from more than 250 years ago: https://www.inaturalist.org/observations/47247203
However, there are some casual observations wrongly determined with earlier dates.