Hi, @Avery_Parks, to answer your questions:
API keys are linked to a specific project. In the API keys tab on the develop.purpleair.com website, you can either create new API keys which are linked to a project, or edit existing keys and change the project they’re linked to. After the keys are linked to a project with points, they should be able to retrieve data without the error you are receiving. Details on adding points to your develop account can be found here: New API Dashboard.
Historical endpoints used to require approval up until the release of the develop site. If you created an account on the develop site yourself, you should have access to the historical endpoints with any new API keys created. Old API keys will still need to be granted access through us. If this is the case, send an email to us with your API keys at contact@purpleair.com.
Also as an additional note, getting data from the historical endpoints on the API has a 2 point base cost per request, as opposed to 1 point for the real-time endpoints. Every field retrieved requires the same amount of points however. Further details on API pricing are available in this article: API Pricing.