This isn’t a syntax error or a bug in the query — it’s Power Query’s Formula Firewall refusing to combine data from more than one source until it knows whether that’s actually safe. Combining a private/organizational source with a public one (an internal database and a public web API, for example) can leak data from one into the other; the firewall blocks it by default rather than guessing.
Why This Exists
Every data source in Power Query has a privacy level — Public, Organizational, or Private — set the first time it’s connected to. When a query’s steps end up needing to send data from one source into a call against a different source, Power Query checks whether the privacy levels involved allow that combination.
Originally published on PBIDocs — Power BI documentation covering DAX, Power Query, data modeling, and Microsoft Fabric.