Hi,
There’s an example on listing all projects in an organization: Get All Projects in an Organization. In that page there is a step-by-step example on how to get all projects in your org.
I think the URI you’re using is not complete. You need to determine your Org-Id and include it in the GET request, e.g.: https://cloud.mongodb.com/api/atlas/v1.0/orgs/{ORG-ID}/groups. Without the Org-Id, it’s an invalid request.
You also need the correct permission set up for the public/private key pair, and the IP whitelist for the API call, otherwise the request will be rejected.
If you need further help, you can use the online chat button in the Atlas page (the button on the lower right hand corner of the screen).
Best regards,
Kevin