First, make sure that https://datastudio.google.com/ is the correct path for the src attribute of the iframe that you are adding. If that is correct, possibly the issue you are running into is that you have added the URL to the allowed iframes setting after you added the iframe to a Discourse post. If that is the case, you will need to click the “Rebuild HTML” button for the post to cause Discourse to pull in the iframe:
If this is for a site that we host, feel free to contact our support team about the issue. We’ll have a look at what’s going on with the iframe.