Just writing to see if anyone else has attempted to use the SharePoint Online WriteBack connector and run into the same problem below - we've currently seen the same issue in 2 separate Domo/SharePoint instances.
After following the steps in the documentation (plus a few extra steps as the documentation is a little thin at the moment) to configure the credentials and progressing to the next step, the connector fails to retrieve (what I'm guessing to be) a list of Site and Group IDs.
No matter the API permissions we grant in the Azure App Registration module we get a "403 forbidden" error. Looking into the response you can see the error message below
Domo Support is aware of this issue, and is currently working with the engineering team to resolve this. If you haven't already reached out, I would submit a ticket if you're able to so they can track the issue.