I succeed to get data with this collector: https://developers.google.com/analytics/devguides/reporting/core/v4/quickstart/service-py
From what I know, bounceRate was used only on UA and it is replaced by EngagementRate on GA4. Yet ga:bounceRate works as a metric while ga:engagementRate does not. Why?
The google analytics reporting api which you are currently using is designed to work only with Universals analytics.
As noted in the overview page
ga:bounceRate works because its is universal analytics ga:engagementRate does not because it is not a Universal analytics metric.
If you are trying to extract data from a GA4 google analytics account then switch to the Google analytics data api