Hi there,
You'll need your developer credentials to query the host API and an AdSense account to act as the publisher.
In order to access publisher data, an Association has to be performed. If you try to access someone else's data, you'll get a 403.
My guess is that you're using your live data IDs but you haven't associated the account.
About your last note: you'll be able to access data but you can't show ads if you're using the host API from a sandbox account. That means that you'll receive empty reports but it's handy in order to check for implementation errors, create ad units, channels, etc.
Cheers,
Jose
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Jose Alcérreca
Developer Relations
Google UK Limited
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