Error getting OData from Microsoft Graph: Access to the resource is forbidden

As @v-qiuyu-msft mentioned earlier, Graph has some custom Auth requirements beyond the usual "Organization Account" login. Even before, most scenarios would encounter this. My own observations from a few months ago were that while the top-level "me" resource and a small number of resources reachable from that worked, the vast majority of resources have always produced this error. Something must have changed on Graph's side for all resources to be producing this error now.

Getting Power BI to use a custom Auth flow is possible by creating a custom Data Connector. We actually have a tutorial specifically for creating a basic sample Graph connector. If you are interested in a dedicated Graph connector, feel free to upvote the idea on UserVoice.