SharePoint 2016 on-prem/Power BI Report Server (on-prem)
In the Power BI desktop client, when you try to 'Get Data' and select the option for SharePoint folder, we enter the URL and trim the path down to the top-level, use Windows (AD) credentials to access the SharePoint content, it returns an error 'Access to the resource is forbidden'. We've tried using a service account specifically set up for this role, and we've tried using our elevated accounts that not only have access, but are site collection admins, and get the same results. I've checked the logs and don't see any errors on the App or Web servers.
@Greg Webb are you trying to get a file or list? I was having the same issue until I specifically selected "sharepoint list" instead of "sharepoint folder" when trying to connect to my sharepoint list.