Hi Royce,
Thanks for your message. Yes, querying with both those dimensions at the same time is not supported. The error message you received is not super helpful, so we will try to improve that - thanks for making us aware of it.
Regarding supported dimensions: as per
https://developers.google.com/adsense/management/reporting/dimensions_metrics#incompatible, if the AdSense UI lets you use a combination of dimensions, then the API should also work with the same combination, so it can be helpful to try a report in the UI first.
Regards,
Andrew (AdSense API team)

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