Do you use the "Dynamics 365 item approval with a more information option" built-in template of Microsoft Flow to create your flow?
The error message told that the Common Data Service database for your current environment of Microsoft Flow has or not supported approvals.
The 429 http error told that you have sent too many requests in a given amount of time. The account is being rate limited. I have made a test on my side and don't have the issue that you mentioned, the "Start an approval" action works well.
Please reduce the request for "Start an approval" action and check if the issue is solved. Please check if the Common Data Serice database of your current environment of Microsoft Flow support approvals.
Community Support Team _ Kris Dai If this post helps, then please consider Accept it as the solution to help the other members find it more quickly.